HTTPbis Working Group M. Belshe
Internet-Draft Twist
Intended status: Standards Track R. Peon
Expires: January 31, 2015 Google, Inc
M. Thomson, Ed.
Mozilla
July 30, 2014
Hypertext Transfer Protocol version 2draft-ietf-httpbis-http2-14
Abstract
This specification describes an optimized expression of the syntax of
the Hypertext Transfer Protocol (HTTP). HTTP/2 enables a more
efficient use of network resources and a reduced perception of
latency by introducing header field compression and allowing multiple
concurrent messages on the same connection. It also introduces
unsolicited push of representations from servers to clients.
This specification is an alternative to, but does not obsolete, the
HTTP/1.1 message syntax. HTTP's existing semantics remain unchanged.
Editorial Note (To be removed by RFC Editor)
Discussion of this draft takes place on the HTTPBIS working group
mailing list (ietf-http-wg@w3.org), which is archived at [1].
Working Group information can be found at [2]; that specific to
HTTP/2 are at [3].
The changes in this draft are summarized in Appendix A.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
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cause the small initial TCP [TCP] congestion window to quickly fill.
This can result in excessive latency when multiple requests are made
on a single new TCP connection.
This specification addresses these issues by defining an optimized
mapping of HTTP's semantics to an underlying connection.
Specifically, it allows interleaving of request and response messages
on the same connection and uses an efficient coding for HTTP header
fields. It also allows prioritization of requests, letting more
important requests complete more quickly, further improving
performance.
The resulting protocol is designed to be more friendly to the
network, because fewer TCP connections can be used in comparison to
HTTP/1.x. This means less competition with other flows, and longer-
lived connections, which in turn leads to better utilization of
available network capacity.
Finally, this encapsulation also enables more efficient processing of
messages through use of binary message framing.
2. HTTP/2 Protocol Overview
HTTP/2 provides an optimized transport for HTTP semantics. HTTP/2
supports all of the core features of HTTP/1.1, but aims to be more
efficient in several ways.
The basic protocol unit in HTTP/2 is a frame (Section 4.1). Each
frame type serves a different purpose. For example, HEADERS and DATA
frames form the basis of HTTP requests and responses (Section 8.1);
other frame types like SETTINGS, WINDOW_UPDATE, and PUSH_PROMISE are
used in support of other HTTP/2 features.
Multiplexing of requests is achieved by having each HTTP request-
response exchanged assigned to a single stream (Section 5). Streams
are largely independent of each other, so a blocked or stalled
request does not prevent progress on other requests.
Flow control and prioritization ensure that it is possible to
properly use multiplexed streams. Flow control (Section 5.2) helps
to ensure that only data that can be used by a receiver is
transmitted. Prioritization (Section 5.3) ensures that limited
resources can be directed to the most important requests first.
HTTP/2 adds a new interaction mode, whereby a server can push
responses to a client (Section 8.2). Server push allows a server to
speculatively send a client data that the server anticipates the
client will need, trading off some network usage against a potential
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latency gain. The server does this by synthesizing a request, which
it sends as a PUSH_PROMISE frame. The server is then able to send a
response to the synthetic request on a separate stream.
Frames that contain HTTP header fields are compressed (Section 4.3).
HTTP requests can be highly redundant, so compression can reduce the
size of requests and responses significantly.
2.1. Document Organization
The HTTP/2 specification is split into four parts:
o Starting HTTP/2 (Section 3) covers how an HTTP/2 connection is
initiated.
o The framing (Section 4) and streams (Section 5) layers describe
the way HTTP/2 frames are structured and formed into multiplexed
streams.
o Frame (Section 6) and error (Section 7) definitions include
details of the frame and error types used in HTTP/2.
o HTTP mappings (Section 8) and additional requirements (Section 9)
describe how HTTP semantics are expressed using frames and
streams.
While some of the frame and stream layer concepts are isolated from
HTTP, the intent is not to define a completely generic framing layer.
The framing and streams layers are tailored to the needs of the HTTP
protocol and server push.
2.2. Conventions and Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
All numeric values are in network byte order. Values are unsigned
unless otherwise indicated. Literal values are provided in decimal
or hexadecimal as appropriate. Hexadecimal literals are prefixed
with "0x" to distinguish them from decimal literals.
The following terms are used:
client: The endpoint initiating the HTTP/2 connection.
connection: A transport-level connection between two endpoints.
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connection error: An error that affects the entire HTTP/2
connection.
endpoint: Either the client or server of the connection.
frame: The smallest unit of communication within an HTTP/2
connection, consisting of a header and a variable-length sequence
of bytes structured according to the frame type.
peer: An endpoint. When discussing a particular endpoint, "peer"
refers to the endpoint that is remote to the primary subject of
discussion.
receiver: An endpoint that is receiving frames.
sender: An endpoint that is transmitting frames.
server: The endpoint which did not initiate the HTTP/2 connection.
stream: A bi-directional flow of frames across a virtual channel
within the HTTP/2 connection.
stream error: An error on the individual HTTP/2 stream.
Finally, the terms "gateway", "intermediary", "proxy", and "tunnel"
are defined in Section 2.3 of [RFC7230].
3. Starting HTTP/2
An HTTP/2 connection is an application level protocol running on top
of a TCP connection ([TCP]). The client is the TCP connection
initiator.
HTTP/2 uses the same "http" and "https" URI schemes used by HTTP/1.1.
HTTP/2 shares the same default port numbers: 80 for "http" URIs and
443 for "https" URIs. As a result, implementations processing
requests for target resource URIs like "http://example.org/foo" or
"https://example.com/bar" are required to first discover whether the
upstream server (the immediate peer to which the client wishes to
establish a connection) supports HTTP/2.
The means by which support for HTTP/2 is determined is different for
"http" and "https" URIs. Discovery for "http" URIs is described in
Section 3.2. Discovery for "https" URIs is described in Section 3.3.
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Internet-Draft HTTP/2 July 20143.1. HTTP/2 Version Identification
The protocol defined in this document has two identifiers.
o The string "h2" identifies the protocol where HTTP/2 uses TLS
[TLS12]. This identifier is used in the TLS application layer
protocol negotiation extension (ALPN) [TLS-ALPN] field and any
place that HTTP/2 over TLS is identified.
The "h2" string is serialized into an ALPN protocol identifier as
the two octet sequence: 0x68, 0x32.
o The string "h2c" identifies the protocol where HTTP/2 is run over
cleartext TCP. This identifier is used in the HTTP/1.1 Upgrade
header field and any place that HTTP/2 over TCP is identified.
Negotiating "h2" or "h2c" implies the use of the transport, security,
framing and message semantics described in this document.
[[CREF1: RFC Editor's Note: please remove the remainder of this
section prior to the publication of a final version of this
document.]]
Only implementations of the final, published RFC can identify
themselves as "h2" or "h2c". Until such an RFC exists,
implementations MUST NOT identify themselves using these strings.
Examples and text throughout the rest of this document use "h2" as a
matter of editorial convenience only. Implementations of draft
versions MUST NOT identify using this string.
Implementations of draft versions of the protocol MUST add the string
"-" and the corresponding draft number to the identifier. For
example, draft-ietf-httpbis-http2-11 over TLS is identified using the
string "h2-11".
Non-compatible experiments that are based on these draft versions
MUST append the string "-" and an experiment name to the identifier.
For example, an experimental implementation of packet mood-based
encoding based on draft-ietf-httpbis-http2-09 might identify itself
as "h2-09-emo". Note that any label MUST conform to the "token"
syntax defined in Section 3.2.6 of [RFC7230]. Experimenters are
encouraged to coordinate their experiments on the ietf-http-wg@w3.org
mailing list.
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Internet-Draft HTTP/2 July 20143.2. Starting HTTP/2 for "http" URIs
A client that makes a request to an "http" URI without prior
knowledge about support for HTTP/2 uses the HTTP Upgrade mechanism
(Section 6.7 of [RFC7230]). The client makes an HTTP/1.1 request
that includes an Upgrade header field identifying HTTP/2 with the
"h2c" token. The HTTP/1.1 request MUST include exactly one
HTTP2-Settings (Section 3.2.1) header field.
For example:
GET / HTTP/1.1
Host: server.example.com
Connection: Upgrade, HTTP2-Settings
Upgrade: h2c
HTTP2-Settings: <base64url encoding of HTTP/2 SETTINGS payload>
Requests that contain an entity body MUST be sent in their entirety
before the client can send HTTP/2 frames. This means that a large
request entity can block the use of the connection until it is
completely sent.
If concurrency of an initial request with subsequent requests is
important, a small request can be used to perform the upgrade to
HTTP/2, at the cost of an additional round-trip.
A server that does not support HTTP/2 can respond to the request as
though the Upgrade header field were absent:
HTTP/1.1 200 OK
Content-Length: 243
Content-Type: text/html
...
A server MUST ignore a "h2" token in an Upgrade header field.
Presence of a token with "h2" implies HTTP/2 over TLS, which is
instead negotiated as described in Section 3.3.
A server that supports HTTP/2 can accept the upgrade with a 101
(Switching Protocols) response. After the empty line that terminates
the 101 response, the server can begin sending HTTP/2 frames. These
frames MUST include a response to the request that initiated the
Upgrade.
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HTTP/1.1 101 Switching Protocols
Connection: Upgrade
Upgrade: h2c
[ HTTP/2 connection ...
The first HTTP/2 frame sent by the server is a SETTINGS frame
(Section 6.5). Upon receiving the 101 response, the client sends a
connection preface (Section 3.5), which includes a SETTINGS frame.
The HTTP/1.1 request that is sent prior to upgrade is assigned stream
identifier 1 and is assigned default priority values (Section 5.3.5).
Stream 1 is implicitly half closed from the client toward the server,
since the request is completed as an HTTP/1.1 request. After
commencing the HTTP/2 connection, stream 1 is used for the response.
3.2.1. HTTP2-Settings Header Field
A request that upgrades from HTTP/1.1 to HTTP/2 MUST include exactly
one "HTTP2-Settings" header field. The "HTTP2-Settings" header field
is a hop-by-hop header field that includes parameters that govern the
HTTP/2 connection, provided in anticipation of the server accepting
the request to upgrade.
HTTP2-Settings = token68
A server MUST reject an attempt to upgrade if this header field is
not present. A server MUST NOT send this header field.
The content of the "HTTP2-Settings" header field is the payload of a
SETTINGS frame (Section 6.5), encoded as a base64url string (that is,
the URL- and filename-safe Base64 encoding described in Section 5 of
[RFC4648], with any trailing '=' characters omitted). The ABNF
[RFC5234] production for "token68" is defined in Section 2.1 of
[RFC7235].
As a hop-by-hop header field, the "Connection" header field MUST
include a value of "HTTP2-Settings" in addition to "Upgrade" when
upgrading to HTTP/2.
A server decodes and interprets these values as it would any other
SETTINGS frame. Acknowledgement of the SETTINGS parameters
(Section 6.5.3) is not necessary, since a 101 response serves as
implicit acknowledgment. Providing these values in the Upgrade
request ensures that the protocol does not require default values for
the above SETTINGS parameters, and gives a client an opportunity to
provide other parameters prior to receiving any frames from the
server.
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Internet-Draft HTTP/2 July 20143.3. Starting HTTP/2 for "https" URIs
A client that makes a request to an "https" URI without prior
knowledge about support for HTTP/2 uses TLS [TLS12] with the
application layer protocol negotiation extension [TLS-ALPN].
HTTP/2 over TLS uses the "h2" application token. The "h2c" token
MUST NOT be sent by a client or selected by a server.
Once TLS negotiation is complete, both the client and the server send
a connection preface (Section 3.5).
3.4. Starting HTTP/2 with Prior Knowledge
A client can learn that a particular server supports HTTP/2 by other
means. For example, [ALT-SVC] describes a mechanism for advertising
this capability.
A client MAY immediately send HTTP/2 frames to a server that is known
to support HTTP/2, after the connection preface (Section 3.5). A
server can identify such a connection by the use of the "PRI" method
in the connection preface. This only affects the establishment of
HTTP/2 connections over cleartext TCP; implementations that support
HTTP/2 over TLS MUST use protocol negotiation in TLS [TLS-ALPN].
Prior support for HTTP/2 is not a strong signal that a given server
will support HTTP/2 for future connections. It is possible for
server configurations to change; for configurations to differ between
instances in clustered server; or network conditions to change.
3.5. HTTP/2 Connection Preface
Upon establishment of a TCP connection and determination that HTTP/2
will be used by both peers, each endpoint MUST send a connection
preface as a final confirmation and to establish the initial SETTINGS
parameters for the HTTP/2 connection.
The client connection preface starts with a sequence of 24 octets,
which in hex notation are:
0x505249202a20485454502f322e300d0a0d0a534d0d0a0d0a
(the string "PRI * HTTP/2.0\r\n\r\nSM\r\n\r\n"). This sequence is
followed by a SETTINGS frame (Section 6.5). The SETTINGS frame MAY
be empty. The client sends the client connection preface immediately
upon receipt of a 101 Switching Protocols response (indicating a
successful upgrade), or as the first application data octets of a TLS
connection. If starting an HTTP/2 connection with prior knowledge of
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server support for the protocol, the client connection preface is
sent upon connection establishment.
The client connection preface is selected so that a large
proportion of HTTP/1.1 or HTTP/1.0 servers and intermediaries do
not attempt to process further frames. Note that this does not
address the concerns raised in [TALKING].
The server connection preface consists of a potentially empty
SETTINGS frame (Section 6.5) that MUST be the first frame the server
sends in the HTTP/2 connection.
To avoid unnecessary latency, clients are permitted to send
additional frames to the server immediately after sending the client
connection preface, without waiting to receive the server connection
preface. It is important to note, however, that the server
connection preface SETTINGS frame might include parameters that
necessarily alter how a client is expected to communicate with the
server. Upon receiving the SETTINGS frame, the client is expected to
honor any parameters established. In some configurations, it is
possible for the server to transmit SETTINGS before the client,
providing an opportunity to avoid this issue.
Clients and servers MUST terminate the TCP connection if either peer
does not begin with a valid connection preface. A GOAWAY frame
(Section 6.8) can be omitted if it is clear that the peer is not
using HTTP/2.
4. HTTP Frames
Once the HTTP/2 connection is established, endpoints can begin
exchanging frames.
4.1. Frame Format
All frames begin with a fixed 9-octet header followed by a variable-
length payload.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length (24) |
+---------------+---------------+---------------+
| Type (8) | Flags (8) |
+-+-+-----------+---------------+-------------------------------+
|R| Stream Identifier (31) |
+=+=============================================================+
| Frame Payload (0...) ...
+---------------------------------------------------------------+
Frame Layout
The fields of the frame header are defined as:
Length: The length of the frame payload expressed as an unsigned
24-bit integer. Values greater than 2^14 (16,384) MUST NOT be
sent unless the receiver has set a larger value for
SETTINGS_MAX_FRAME_SIZE.
The 9 octets of the frame header are not included in this value.
Type: The 8-bit type of the frame. The frame type determines the
format and semantics of the frame. Implementations MUST ignore
and discard any frame that has a type that is unknown.
Flags: An 8-bit field reserved for frame-type specific boolean
flags.
Flags are assigned semantics specific to the indicated frame type.
Flags that have no defined semantics for a particular frame type
MUST be ignored, and MUST be left unset (0) when sending.
R: A reserved 1-bit field. The semantics of this bit are undefined
and the bit MUST remain unset (0) when sending and MUST be ignored
when receiving.
Stream Identifier: A 31-bit stream identifier (see Section 5.1.1).
The value 0 is reserved for frames that are associated with the
connection as a whole as opposed to an individual stream.
The structure and content of the frame payload is dependent entirely
on the frame type.
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Internet-Draft HTTP/2 July 20144.2. Frame Size
The size of a frame payload is limited by the maximum size that a
receiver advertises in the SETTINGS_MAX_FRAME_SIZE setting. This
setting can have any value between 2^14 (16,384) and 2^24-1
(16,777,215) octets, inclusive.
All implementations MUST be capable of receiving and minimally
processing frames up to 2^14 octets in length, plus the 9 octet frame
header (Section 4.1). The size of the frame header is not included
when describing frame sizes.
Note: Certain frame types, such as PING (Section 6.7), impose
additional limits on the amount of payload data allowed.
If a frame size exceeds any defined limit, or is too small to contain
mandatory frame data, the endpoint MUST send a FRAME_SIZE_ERROR
error. A frame size error in a frame that could alter the state of
the entire connection MUST be treated as a connection error
(Section 5.4.1); this includes any frame carrying a header block
(Section 4.3) (that is, HEADERS, PUSH_PROMISE, and CONTINUATION),
SETTINGS, and any WINDOW_UPDATE frame with a stream identifier of 0.
Endpoints are not obligated to use all available space in a frame.
Responsiveness can be improved by using frames that are smaller than
the permitted maximum size. Sending large frames can result in
delays in sending maintenance frames, such RST_STREAM, WINDOW_UPDATE,
or PRIORITY, which if blocked by the transmission of a large frame,
could affect performance.
4.3. Header Compression and Decompression
A header field in HTTP/2 is a name with one or more associated
values. They are used within HTTP request and response messages as
well as server push operations (see Section 8.2).
Header lists are collections of zero or more header fields. When
transmitted over a connection, a header list is serialized into a
header block using HTTP Header Compression [COMPRESSION]. The
serialized header block is then divided into one or more octet
sequences, called header block fragments, and transmitted within the
payload of HEADERS (Section 6.2), PUSH_PROMISE (Section 6.6) or
CONTINUATION (Section 6.10) frames.
The Cookie header field [COOKIE] is treated specially by the HTTP
mapping (see Section 8.1.2.5).
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A receiving endpoint reassembles the header block by concatenating
its fragments, then decompresses the block to reconstruct the header
list.
A complete header block consists of either:
o a single HEADERS or PUSH_PROMISE frame, with the END_HEADERS flag
set, or
o a HEADERS or PUSH_PROMISE frame with the END_HEADERS flag cleared
and one or more CONTINUATION frames, where the last CONTINUATION
frame has the END_HEADERS flag set.
Header compression is stateful, using a single compression context
for the entire connection. Each header block is processed as a
discrete unit. Header blocks MUST be transmitted as a contiguous
sequence of frames, with no interleaved frames of any other type or
from any other stream. The last frame in a sequence of HEADERS or
CONTINUATION frames MUST have the END_HEADERS flag set. The last
frame in a sequence of PUSH_PROMISE or CONTINUATION frames MUST have
the END_HEADERS flag set. This allows a header block to be logically
equivalent to a single frame.
Header block fragments can only be sent as the payload of HEADERS,
PUSH_PROMISE or CONTINUATION frames, because these frames carry data
that can modify the compression context maintained by a receiver. An
endpoint receiving HEADERS, PUSH_PROMISE or CONTINUATION frames MUST
reassemble header blocks and perform decompression even if the frames
are to be discarded. A receiver MUST terminate the connection with a
connection error (Section 5.4.1) of type COMPRESSION_ERROR if it does
not decompress a header block.
5. Streams and Multiplexing
A "stream" is an independent, bi-directional sequence of frames
exchanged between the client and server within an HTTP/2 connection.
Streams have several important characteristics:
o A single HTTP/2 connection can contain multiple concurrently open
streams, with either endpoint interleaving frames from multiple
streams.
o Streams can be established and used unilaterally or shared by
either the client or server.
o Streams can be closed by either endpoint.
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Note that this diagram shows stream state transitions and frames that
affect those transitions only. In this regard, CONTINUATION frames
do not result in state transitions and are effectively part of the
HEADERS or PUSH_PROMISE that they follow.
Both endpoints have a subjective view of the state of a stream that
could be different when frames are in transit. Endpoints do not
coordinate the creation of streams; they are created unilaterally by
either endpoint. The negative consequences of a mismatch in states
are limited to the "closed" state after sending RST_STREAM, where
frames might be received for some time after closing.
Streams have the following states:
idle:
All streams start in the "idle" state. In this state, no frames
have been exchanged.
The following transitions are valid from this state:
* Sending or receiving a HEADERS frame causes the stream to
become "open". The stream identifier is selected as described
in Section 5.1.1. The same HEADERS frame can also cause a
stream to immediately become "half closed".
* Sending a PUSH_PROMISE frame marks the associated stream for
later use. The stream state for the reserved stream
transitions to "reserved (local)".
* Receiving a PUSH_PROMISE frame marks the associated stream as
reserved by the remote peer. The state of the stream becomes
"reserved (remote)".
reserved (local):
A stream in the "reserved (local)" state is one that has been
promised by sending a PUSH_PROMISE frame. A PUSH_PROMISE frame
reserves an idle stream by associating the stream with an open
stream that was initiated by the remote peer (see Section 8.2).
In this state, only the following transitions are possible:
* The endpoint can send a HEADERS frame. This causes the stream
to open in a "half closed (remote)" state.
* Either endpoint can send a RST_STREAM frame to cause the stream
to become "closed". This releases the stream reservation.
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An endpoint MUST NOT send frames other than HEADERS or RST_STREAM
in this state.
A PRIORITY frame MAY be received in this state. Receiving any
frames other than RST_STREAM, or PRIORITY MUST be treated as a
connection error (Section 5.4.1) of type PROTOCOL_ERROR.
reserved (remote):
A stream in the "reserved (remote)" state has been reserved by a
remote peer.
In this state, only the following transitions are possible:
* Receiving a HEADERS frame causes the stream to transition to
"half closed (local)".
* Either endpoint can send a RST_STREAM frame to cause the stream
to become "closed". This releases the stream reservation.
An endpoint MAY send a PRIORITY frame in this state to
reprioritize the reserved stream. An endpoint MUST NOT send any
other type of frame other than RST_STREAM or PRIORITY.
Receiving any other type of frame other than HEADERS or RST_STREAM
MUST be treated as a connection error (Section 5.4.1) of type
PROTOCOL_ERROR.
open:
A stream in the "open" state may be used by both peers to send
frames of any type. In this state, sending peers observe
advertised stream level flow control limits (Section 5.2).
From this state either endpoint can send a frame with an
END_STREAM flag set, which causes the stream to transition into
one of the "half closed" states: an endpoint sending an END_STREAM
flag causes the stream state to become "half closed (local)"; an
endpoint receiving an END_STREAM flag causes the stream state to
become "half closed (remote)".
Either endpoint can send a RST_STREAM frame from this state,
causing it to transition immediately to "closed".
half closed (local):
A stream that is in the "half closed (local)" state cannot be used
for sending frames. Only WINDOW_UPDATE, PRIORITY and RST_STREAM
frames can be sent in this state.
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A stream transitions from this state to "closed" when a frame that
contains an END_STREAM flag is received, or when either peer sends
a RST_STREAM frame.
A receiver can ignore WINDOW_UPDATE frames in this state, which
might arrive for a short period after a frame bearing the
END_STREAM flag is sent.
PRIORITY frames received in this state are used to reprioritize
streams that depend on the current stream.
half closed (remote):
A stream that is "half closed (remote)" is no longer being used by
the peer to send frames. In this state, an endpoint is no longer
obligated to maintain a receiver flow control window if it
performs flow control.
If an endpoint receives additional frames for a stream that is in
this state, other than WINDOW_UPDATE, PRIORITY or RST_STREAM, it
MUST respond with a stream error (Section 5.4.2) of type
STREAM_CLOSED.
A stream that is "half closed (remote)" can be used by the
endpoint to send frames of any type. In this state, the endpoint
continues to observe advertised stream level flow control limits
(Section 5.2).
A stream can transition from this state to "closed" by sending a
frame that contains an END_STREAM flag, or when either peer sends
a RST_STREAM frame.
closed:
The "closed" state is the terminal state.
An endpoint MUST NOT send frames on a closed stream. An endpoint
that receives any frame other than PRIORITY after receiving a
RST_STREAM MUST treat that as a stream error (Section 5.4.2) of
type STREAM_CLOSED. Similarly, an endpoint that receives any
frames after receiving a frame with the END_STREAM flag set MUST
treat that as a connection error (Section 5.4.1) of type
STREAM_CLOSED, unless the frame is permitted as described below.
WINDOW_UPDATE or RST_STREAM frames can be received in this state
for a short period after a DATA or HEADERS frame containing an
END_STREAM flag is sent. Until the remote peer receives and
processes the frame bearing the END_STREAM flag, it might send
frames of these types. Endpoints MUST ignore WINDOW_UPDATE or
RST_STREAM frames received in this state, though endpoints MAY
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choose to treat frames that arrive a significant time after
sending END_STREAM as a connection error (Section 5.4.1) of type
PROTOCOL_ERROR.
PRIORITY frames can be sent on closed streams to prioritize
streams that are dependent on the closed stream. Endpoints SHOULD
process PRIORITY frame, though they can be ignored if the stream
has been removed from the dependency tree (see Section 5.3.4).
If this state is reached as a result of sending a RST_STREAM
frame, the peer that receives the RST_STREAM might have already
sent - or enqueued for sending - frames on the stream that cannot
be withdrawn. An endpoint MUST ignore frames that it receives on
closed streams after it has sent a RST_STREAM frame. An endpoint
MAY choose to limit the period over which it ignores frames and
treat frames that arrive after this time as being in error.
Flow controlled frames (i.e., DATA) received after sending
RST_STREAM are counted toward the connection flow control window.
Even though these frames might be ignored, because they are sent
before the sender receives the RST_STREAM, the sender will
consider the frames to count against the flow control window.
An endpoint might receive a PUSH_PROMISE frame after it sends
RST_STREAM. PUSH_PROMISE causes a stream to become "reserved"
even if the associated stream has been reset. Therefore, a
RST_STREAM is needed to close an unwanted promised stream.
In the absence of more specific guidance elsewhere in this document,
implementations SHOULD treat the receipt of a message that is not
expressly permitted in the description of a state as a connection
error (Section 5.4.1) of type PROTOCOL_ERROR.
An example of the state transitions for an HTTP request/response
exchange can be found in Section 8.1. An example of the state
transitions for server push can be found in Section 8.2.1 and
Section 8.2.2.
5.1.1. Stream Identifiers
Streams are identified with an unsigned 31-bit integer. Streams
initiated by a client MUST use odd-numbered stream identifiers; those
initiated by the server MUST use even-numbered stream identifiers. A
stream identifier of zero (0x0) is used for connection control
messages; the stream identifier zero cannot be used to establish a
new stream.
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HTTP/1.1 requests that are upgraded to HTTP/2 (see Section 3.2) are
responded to with a stream identifier of one (0x1). After the
upgrade completes, stream 0x1 is "half closed (local)" to the client.
Therefore, stream 0x1 cannot be selected as a new stream identifier
by a client that upgrades from HTTP/1.1.
The identifier of a newly established stream MUST be numerically
greater than all streams that the initiating endpoint has opened or
reserved. This governs streams that are opened using a HEADERS frame
and streams that are reserved using PUSH_PROMISE. An endpoint that
receives an unexpected stream identifier MUST respond with a
connection error (Section 5.4.1) of type PROTOCOL_ERROR.
The first use of a new stream identifier implicitly closes all
streams in the "idle" state that might have been initiated by that
peer with a lower-valued stream identifier. For example, if a client
sends a HEADERS frame on stream 7 without ever sending a frame on
stream 5, then stream 5 transitions to the "closed" state when the
first frame for stream 7 is sent or received.
Stream identifiers cannot be reused. Long-lived connections can
result in an endpoint exhausting the available range of stream
identifiers. A client that is unable to establish a new stream
identifier can establish a new connection for new streams. A server
that is unable to establish a new stream identifier can send a GOAWAY
frame so that the client is forced to open a new connection for new
streams.
5.1.2. Stream Concurrency
A peer can limit the number of concurrently active streams using the
SETTINGS_MAX_CONCURRENT_STREAMS parameter (see Section 6.5.2) within
a SETTINGS frame. The maximum concurrent streams setting is specific
to each endpoint and applies only to the peer that receives the
setting. That is, clients specify the maximum number of concurrent
streams the server can initiate, and servers specify the maximum
number of concurrent streams the client can initiate.
Streams that are in the "open" state, or either of the "half closed"
states count toward the maximum number of streams that an endpoint is
permitted to open. Streams in any of these three states count toward
the limit advertised in the SETTINGS_MAX_CONCURRENT_STREAMS setting.
Streams in either of the "reserved" states do not count toward the
stream limit.
Endpoints MUST NOT exceed the limit set by their peer. An endpoint
that receives a HEADERS frame that causes their advertised concurrent
stream limit to be exceeded MUST treat this as a stream error
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(Section 5.4.2). An endpoint that wishes to reduce the value of
SETTINGS_MAX_CONCURRENT_STREAMS to a value that is below the current
number of open streams can either close streams that exceed the new
value or allow streams to complete.
5.2. Flow Control
Using streams for multiplexing introduces contention over use of the
TCP connection, resulting in blocked streams. A flow control scheme
ensures that streams on the same connection do not destructively
interfere with each other. Flow control is used for both individual
streams and for the connection as a whole.
HTTP/2 provides for flow control through use of the WINDOW_UPDATE
frame (Section 6.9).
5.2.1. Flow Control Principles
HTTP/2 stream flow control aims to allow for future improvements to
flow control algorithms without requiring protocol changes. Flow
control in HTTP/2 has the following characteristics:
1. Flow control is hop-by-hop, not end-to-end.
2. Flow control is based on window update frames. Receivers
advertise how many bytes they are prepared to receive on a stream
and for the entire connection. This is a credit-based scheme.
3. Flow control is directional with overall control provided by the
receiver. A receiver MAY choose to set any window size that it
desires for each stream and for the entire connection. A sender
MUST respect flow control limits imposed by a receiver. Clients,
servers and intermediaries all independently advertise their flow
control window as a receiver and abide by the flow control limits
set by their peer when sending.
4. The initial value for the flow control window is 65,535 bytes for
both new streams and the overall connection.
5. The frame type determines whether flow control applies to a
frame. Of the frames specified in this document, only DATA
frames are subject to flow control; all other frame types do not
consume space in the advertised flow control window. This
ensures that important control frames are not blocked by flow
control.
6. Flow control cannot be disabled.
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7. HTTP/2 defines only the format and semantics of the WINDOW_UPDATE
frame (Section 6.9). This document does not stipulate how a
receiver decides when to send this frame or the value that it
sends. Nor does it specify how a sender chooses to send packets.
Implementations are able to select any algorithm that suits their
needs.
Implementations are also responsible for managing how requests and
responses are sent based on priority; choosing how to avoid head of
line blocking for requests; and managing the creation of new streams.
Algorithm choices for these could interact with any flow control
algorithm.
5.2.2. Appropriate Use of Flow Control
Flow control is defined to protect endpoints that are operating under
resource constraints. For example, a proxy needs to share memory
between many connections, and also might have a slow upstream
connection and a fast downstream one. Flow control addresses cases
where the receiver is unable process data on one stream, yet wants to
continue to process other streams in the same connection.
Deployments that do not require this capability can advertise a flow
control window of the maximum size, incrementing the available space
when new data is received. This effectively disables flow control
for that receiver. Conversely, a sender is always subject to the
flow control window advertised by the receiver.
Deployments with constrained resources (for example, memory) can
employ flow control to limit the amount of memory a peer can consume.
Note, however, that this can lead to suboptimal use of available
network resources if flow control is enabled without knowledge of the
bandwidth-delay product (see [RFC1323]).
Even with full awareness of the current bandwidth-delay product,
implementation of flow control can be difficult. When using flow
control, the receiver MUST read from the TCP receive buffer in a
timely fashion. Failure to do so could lead to a deadlock when
critical frames, such as WINDOW_UPDATE, are not read and acted upon.
5.3. Stream priority
A client can assign a priority for a new stream by including
prioritization information in the HEADERS frame (Section 6.2) that
opens the stream. For an existing stream, the PRIORITY frame
(Section 6.3) can be used to change the priority.
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The purpose of prioritization is to allow an endpoint to express how
it would prefer its peer allocate resources when managing concurrent
streams. Most importantly, priority can be used to select streams
for transmitting frames when there is limited capacity for sending.
Streams can be prioritized by marking them as dependent on the
completion of other streams (Section 5.3.1). Each dependency is
assigned a relative weight, a number that is used to determine the
relative proportion of available resources that are assigned to
streams dependent on the same stream.
Explicitly setting the priority for a stream is input to a
prioritization process. It does not guarantee any particular
processing or transmission order for the stream relative to any other
stream. An endpoint cannot force a peer to process concurrent
streams in a particular order using priority. Expressing priority is
therefore only ever a suggestion.
Prioritization information can be specified explicitly for streams as
they are created using the HEADERS frame, or changed using the
PRIORITY frame. Providing prioritization information is optional, so
default values are used if no explicit indicator is provided
(Section 5.3.5).
5.3.1. Stream Dependencies
Each stream can be given an explicit dependency on another stream.
Including a dependency expresses a preference to allocate resources
to the identified stream rather than to the dependent stream.
A stream that is not dependent on any other stream is given a stream
dependency of 0x0. In other words, the non-existent stream 0 forms
the root of the tree.
A stream that depends on another stream is a dependent stream. The
stream upon which a stream is dependent is a parent stream. A
dependency on a stream that is not currently in the tree - such as a
stream in the "idle" state - results in the stream being given a
default priority (Section 5.3.5).
When assigning a dependency on another stream, the stream is added as
a new dependency of the parent stream. Dependent streams that share
the same parent are not ordered with respect to each other. For
example, if streams B and C are dependent on stream A, and if stream
D is created with a dependency on stream A, this results in a
dependency order of A followed by B, C, and D in any order.
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A A
/ \ ==> /|\
B C B D C
Example of Default Dependency Creation
An exclusive flag allows for the insertion of a new level of
dependencies. The exclusive flag causes the stream to become the
sole dependency of its parent stream, causing other dependencies to
become dependent on the prioritized stream. In the previous example,
if stream D is created with an exclusive dependency on stream A, this
results in D becoming the dependency parent of B and C.
A
A |
/ \ ==> D
B C / \
B C
Example of Exclusive Dependency Creation
Inside the dependency tree, a dependent stream SHOULD only be
allocated resources if all of the streams that it depends on (the
chain of parent streams up to 0x0) are either closed, or it is not
possible to make progress on them.
A stream cannot depend on itself. An endpoint MUST treat this as a
stream error (Section 5.4.2) of type PROTOCOL_ERROR.
5.3.2. Dependency Weighting
All dependent streams are allocated an integer weight between 1 to
256 (inclusive).
Streams with the same parent SHOULD be allocated resources
proportionally based on their weight. Thus, if stream B depends on
stream A with weight 4, and C depends on stream A with weight 12, and
if no progress can be made on A, stream B ideally receives one third
of the resources allocated to stream C.
5.3.3. Reprioritization
Stream priorities are changed using the PRIORITY frame. Setting a
dependency causes a stream to become dependent on the identified
parent stream.
Dependent streams move with their parent stream if the parent is
reprioritized. Setting a dependency with the exclusive flag for a
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reprioritized stream moves all the dependencies of the new parent
stream to become dependent on the reprioritized stream.
If a stream is made dependent on one of its own dependencies, the
formerly dependent stream is first moved to be dependent on the
reprioritized stream's previous parent. The moved dependency retains
its weight.
For example, consider an original dependency tree where B and C
depend on A, D and E depend on C, and F depends on D. If A is made
dependent on D, then D takes the place of A. All other dependency
relationships stay the same, except for F, which becomes dependent on
A if the reprioritization is exclusive.
? ? ? ?
| / \ | |
A D A D D
/ \ / / \ / \ |
B C ==> F B C ==> F A OR A
/ \ | / \ /|\
D E E B C B C F
| | |
F E E
(intermediate) (non-exclusive) (exclusive)
Example of Dependency Reordering
5.3.4. Prioritization State Management
When a stream is removed from the dependency tree, its dependencies
can be moved to become dependent on the parent of the closed stream.
The weights of new dependencies are recalculated by distributing the
weight of the dependency of the closed stream proportionally based on
the weights of its dependencies.
Streams that are removed from the dependency tree cause some
prioritization information to be lost. Resources are shared between
streams with the same parent stream, which means that if a stream in
that set closes or becomes blocked, any spare capacity allocated to a
stream is distributed to the immediate neighbors of the stream.
However, if the common dependency is removed from the tree, those
streams share resources with streams at the next highest level.
For example, assume streams A and B share a parent, and streams C and
D both depend on stream A. Prior to the removal of stream A, if
streams A and D are unable to proceed, then stream C receives all the
resources dedicated to stream A. If stream A is removed from the
tree, the weight of stream A is divided between streams C and D. If
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stream D is still unable to proceed, this results in stream C
receiving a reduced proportion of resources. For equal starting
weights, C receives one third, rather than one half, of available
resources.
It is possible for a stream to become closed while prioritization
information that creates a dependency on that stream is in transit.
If a stream identified in a dependency has had any associated
priority information destroyed, then the dependent stream is instead
assigned a default priority. This potentially creates suboptimal
prioritization, since the stream could be given a priority that is
higher than intended.
To avoid these problems, an endpoint SHOULD retain stream
prioritization state for a period after streams become closed. The
longer state is retained, the lower the chance that streams are
assigned incorrect or default priority values.
This could create a large state burden for an endpoint, so this state
MAY be limited. An endpoint MAY apply a fixed upper limit on the
number of closed streams for which prioritization state is tracked to
limit state exposure. The amount of additional state an endpoint
maintains could be dependent on load; under high load, prioritization
state can be discarded to limit resource commitments. In extreme
cases, an endpoint could even discard prioritization state for active
or reserved streams. If a fixed limit is applied, endpoints SHOULD
maintain state for at least as many streams as allowed by their
setting for SETTINGS_MAX_CONCURRENT_STREAMS.
An endpoint receiving a PRIORITY frame that changes the priority of a
closed stream SHOULD alter the dependencies of the streams that
depend on it, if it has retained enough state to do so.
5.3.5. Default Priorities
Providing priority information is optional. Streams are assigned a
non-exclusive dependency on stream 0x0 by default. Pushed streams
(Section 8.2) initially depend on their associated stream. In both
cases, streams are assigned a default weight of 16.
5.4. Error Handling
HTTP/2 framing permits two classes of error:
o An error condition that renders the entire connection unusable is
a connection error.
o An error in an individual stream is a stream error.
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A list of error codes is included in Section 7.
5.4.1. Connection Error Handling
A connection error is any error which prevents further processing of
the framing layer, or which corrupts any connection state.
An endpoint that encounters a connection error SHOULD first send a
GOAWAY frame (Section 6.8) with the stream identifier of the last
stream that it successfully received from its peer. The GOAWAY frame
includes an error code that indicates why the connection is
terminating. After sending the GOAWAY frame, the endpoint MUST close
the TCP connection.
It is possible that the GOAWAY will not be reliably received by the
receiving endpoint. In the event of a connection error, GOAWAY only
provides a best effort attempt to communicate with the peer about why
the connection is being terminated.
An endpoint can end a connection at any time. In particular, an
endpoint MAY choose to treat a stream error as a connection error.
Endpoints SHOULD send a GOAWAY frame when ending a connection,
providing that circumstances permit it.
5.4.2. Stream Error Handling
A stream error is an error related to a specific stream that does not
affect processing of other streams.
An endpoint that detects a stream error sends a RST_STREAM frame
(Section 6.4) that contains the stream identifier of the stream where
the error occurred. The RST_STREAM frame includes an error code that
indicates the type of error.
A RST_STREAM is the last frame that an endpoint can send on a stream.
The peer that sends the RST_STREAM frame MUST be prepared to receive
any frames that were sent or enqueued for sending by the remote peer.
These frames can be ignored, except where they modify connection
state (such as the state maintained for header compression
(Section 4.3), or flow control).
Normally, an endpoint SHOULD NOT send more than one RST_STREAM frame
for any stream. However, an endpoint MAY send additional RST_STREAM
frames if it receives frames on a closed stream after more than a
round-trip time. This behavior is permitted to deal with misbehaving
implementations.
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An endpoint MUST NOT send a RST_STREAM in response to an RST_STREAM
frame, to avoid looping.
5.4.3. Connection Termination
If the TCP connection is torn down while streams remain in open or
half closed states, then the endpoint MUST assume that those streams
were abnormally interrupted and could be incomplete.
5.5. Extending HTTP/2
HTTP/2 permits extension of the protocol. Protocol extensions can be
used to provide additional services or alter any aspect of the
protocol, within the limitations described in this section.
Extensions are effective only within the scope of a single HTTP/2
connection.
Extensions are permitted to use new frame types (Section 4.1), new
settings (Section 6.5.2), or new error codes (Section 7). Registries
are established for managing these extension points: frame types
(Section 11.2), settings (Section 11.3) and error codes
(Section 11.4).
Implementations MUST ignore unknown or unsupported values in all
extensible protocol elements. Implementations MUST discard frames
that have unknown or unsupported types. This means that any of these
extension points can be safely used by extensions without prior
arrangement or negotiation.
However, extensions that could change the semantics of existing
protocol components MUST be negotiated before being used. For
example, an extension that changes the layout of the HEADERS frame
cannot be used until the peer has given a positive signal that this
is acceptable. In this case, it could also be necessary to
coordinate when the revised layout comes into effect. Note that
treating any frame other than DATA frames as flow controlled is such
a change in semantics, and can only be done through negotiation.
This document doesn't mandate a specific method for negotiating the
use of an extension, but notes that a setting (Section 6.5.2) could
be used for that purpose. If both peers set a value that indicates
willingness to use the extension, then the extension can be used. If
a setting is used for extension negotiation, the initial value MUST
be defined so that the extension is initially disabled.
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This specification defines a number of frame types, each identified
by a unique 8-bit type code. Each frame type serves a distinct
purpose either in the establishment and management of the connection
as a whole, or of individual streams.
The transmission of specific frame types can alter the state of a
connection. If endpoints fail to maintain a synchronized view of the
connection state, successful communication within the connection will
no longer be possible. Therefore, it is important that endpoints
have a shared comprehension of how the state is affected by the use
any given frame.
6.1. DATA
DATA frames (type=0x0) convey arbitrary, variable-length sequences of
octets associated with a stream. One or more DATA frames are used,
for instance, to carry HTTP request or response payloads.
DATA frames MAY also contain arbitrary padding. Padding can be added
to DATA frames to obscure the size of messages.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Pad Length? (8)|
+---------------+-----------------------------------------------+
| Data (*) ...
+---------------------------------------------------------------+
| Padding (*) ...
+---------------------------------------------------------------+
DATA Frame Payload
The DATA frame contains the following fields:
Pad Length: An 8-bit field containing the length of the frame
padding in units of octets. This field is optional and is only
present if the PADDED flag is set.
Data: Application data. The amount of data is the remainder of the
frame payload after subtracting the length of the other fields
that are present.
Padding: Padding octets that contain no application semantic value.
Padding octets MUST be set to zero when sending and ignored when
receiving.
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The DATA frame defines the following flags:
END_STREAM (0x1): Bit 1 being set indicates that this frame is the
last that the endpoint will send for the identified stream.
Setting this flag causes the stream to enter one of the "half
closed" states or the "closed" state (Section 5.1).
PADDED (0x8): Bit 4 being set indicates that the Pad Length field is
present.
DATA frames MUST be associated with a stream. If a DATA frame is
received whose stream identifier field is 0x0, the recipient MUST
respond with a connection error (Section 5.4.1) of type
PROTOCOL_ERROR.
DATA frames are subject to flow control and can only be sent when a
stream is in the "open" or "half closed (remote)" states. The entire
DATA frame payload is included in flow control, including Pad Length
and Padding fields if present. If a DATA frame is received whose
stream is not in "open" or "half closed (local)" state, the recipient
MUST respond with a stream error (Section 5.4.2) of type
STREAM_CLOSED.
The total number of padding octets is determined by the value of the
Pad Length field. If the length of the padding is greater than the
length of the remainder of the frame payload, the recipient MUST
treat this as a connection error (Section 5.4.1) of type
PROTOCOL_ERROR.
Note: A frame can be increased in size by one octet by including a
Pad Length field with a value of zero.
Use of padding is a security feature; as such, its use demands some
care, see Section 10.7.
6.2. HEADERS
The HEADERS frame (type=0x1) carries name-value pairs. It is used to
open a stream (Section 5.1). HEADERS frames can be sent on a stream
in the "open" or "half closed (remote)" states.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Pad Length? (8)|
+-+-------------+-----------------------------------------------+
|E| Stream Dependency? (31) |
+-+-------------+-----------------------------------------------+
| Weight? (8) |
+-+-------------+-----------------------------------------------+
| Header Block Fragment (*) ...
+---------------------------------------------------------------+
| Padding (*) ...
+---------------------------------------------------------------+
HEADERS Frame Payload
The HEADERS frame payload has the following fields:
Pad Length: An 8-bit field containing the length of the frame
padding in units of octets. This field is optional and is only
present if the PADDED flag is set.
E: A single bit flag indicates that the stream dependency is
exclusive, see Section 5.3. This field is optional and is only
present if the PRIORITY flag is set.
Stream Dependency: A 31-bit stream identifier for the stream that
this stream depends on, see Section 5.3. This field is optional
and is only present if the PRIORITY flag is set.
Weight: An 8-bit weight for the stream, see Section 5.3. Add one to
the value to obtain a weight between 1 and 256. This field is
optional and is only present if the PRIORITY flag is set.
Header Block Fragment: A header block fragment (Section 4.3).
Padding: Padding octets.
The HEADERS frame defines the following flags:
END_STREAM (0x1): Bit 1 being set indicates that the header block
(Section 4.3) is the last that the endpoint will send for the
identified stream. Setting this flag causes the stream to enter
one of "half closed" states (Section 5.1).
A HEADERS frame that is followed by CONTINUATION frames carries
the END_STREAM flag that signals the end of a stream. A
CONTINUATION frame cannot be used to terminate a stream.
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END_HEADERS (0x4): Bit 3 being set indicates that this frame
contains an entire header block (Section 4.3) and is not followed
by any CONTINUATION frames.
A HEADERS frame without the END_HEADERS flag set MUST be followed
by a CONTINUATION frame for the same stream. A receiver MUST
treat the receipt of any other type of frame or a frame on a
different stream as a connection error (Section 5.4.1) of type
PROTOCOL_ERROR.
PADDED (0x8): Bit 4 being set indicates that the Pad Length field is
present.
PRIORITY (0x20): Bit 6 being set indicates that the Exclusive Flag
(E), Stream Dependency, and Weight fields are present; see
Section 5.3.
The payload of a HEADERS frame contains a header block fragment
(Section 4.3). A header block that does not fit within a HEADERS
frame is continued in a CONTINUATION frame (Section 6.10).
HEADERS frames MUST be associated with a stream. If a HEADERS frame
is received whose stream identifier field is 0x0, the recipient MUST
respond with a connection error (Section 5.4.1) of type
PROTOCOL_ERROR.
The HEADERS frame changes the connection state as described in
Section 4.3.
The HEADERS frame includes optional padding. Padding fields and
flags are identical to those defined for DATA frames (Section 6.1).
6.3. PRIORITY
The PRIORITY frame (type=0x2) specifies the sender-advised priority
of a stream (Section 5.3). It can be sent at any time for an
existing stream, including closed streams. This enables
reprioritization of existing streams.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|E| Stream Dependency (31) |
+-+-------------+-----------------------------------------------+
| Weight (8) |
+-+-------------+
PRIORITY Frame Payload
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The payload of a PRIORITY frame contains the following fields:
E: A single bit flag indicates that the stream dependency is
exclusive, see Section 5.3.
Stream Dependency: A 31-bit stream identifier for the stream that
this stream depends on, see Section 5.3.
Weight: An 8-bit weight for the identified stream dependency, see
Section 5.3. Add one to the value to obtain a weight between 1
and 256.
The PRIORITY frame does not define any flags.
The PRIORITY frame is associated with an existing stream. If a
PRIORITY frame is received with a stream identifier of 0x0, the
recipient MUST respond with a connection error (Section 5.4.1) of
type PROTOCOL_ERROR.
The PRIORITY frame can be sent on a stream in any of the "reserved
(remote)", "open", "half closed (local)", "half closed (remote)", or
"closed" states, though it cannot be sent between consecutive frames
that comprise a single header block (Section 4.3). Note that this
frame could arrive after processing or frame sending has completed,
which would cause it to have no effect on the current stream. For a
stream that is in the "half closed (remote)" or "closed" - state,
this frame can only affect processing of the current stream and not
frame transmission.
The PRIORITY frame is the only frame that can be sent for a stream in
the "closed" state. This allows for the reprioritization of a group
of dependent streams by altering the priority of a parent stream,
which might be closed. However, a PRIORITY frame sent on a closed
stream risks being ignored due to the peer having discarded priority
state information for that stream.
6.4. RST_STREAM
The RST_STREAM frame (type=0x3) allows for abnormal termination of a
stream. When sent by the initiator of a stream, it indicates that
they wish to cancel the stream or that an error condition has
occurred. When sent by the receiver of a stream, it indicates that
either the receiver is rejecting the stream, requesting that the
stream be cancelled, or that an error condition has occurred.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Error Code (32) |
+---------------------------------------------------------------+
RST_STREAM Frame Payload
The RST_STREAM frame contains a single unsigned, 32-bit integer
identifying the error code (Section 7). The error code indicates why
the stream is being terminated.
The RST_STREAM frame does not define any flags.
The RST_STREAM frame fully terminates the referenced stream and
causes it to enter the closed state. After receiving a RST_STREAM on
a stream, the receiver MUST NOT send additional frames for that
stream. However, after sending the RST_STREAM, the sending endpoint
MUST be prepared to receive and process additional frames sent on the
stream that might have been sent by the peer prior to the arrival of
the RST_STREAM.
RST_STREAM frames MUST be associated with a stream. If a RST_STREAM
frame is received with a stream identifier of 0x0, the recipient MUST
treat this as a connection error (Section 5.4.1) of type
PROTOCOL_ERROR.
RST_STREAM frames MUST NOT be sent for a stream in the "idle" state.
If a RST_STREAM frame identifying an idle stream is received, the
recipient MUST treat this as a connection error (Section 5.4.1) of
type PROTOCOL_ERROR.
6.5. SETTINGS
The SETTINGS frame (type=0x4) conveys configuration parameters that
affect how endpoints communicate, such as preferences and constraints
on peer behavior. The SETTINGS frame is also used to acknowledge the
receipt of those parameters. Individually, a SETTINGS parameter can
also be referred to as a "setting".
SETTINGS parameters are not negotiated; they describe characteristics
of the sending peer, which are used by the receiving peer. Different
values for the same parameter can be advertised by each peer. For
example, a client might set a high initial flow control window,
whereas a server might set a lower value to conserve resources.
A SETTINGS frame MUST be sent by both endpoints at the start of a
connection, and MAY be sent at any other time by either endpoint over
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the lifetime of the connection. Implementations MUST support all of
the parameters defined by this specification.
Each parameter in a SETTINGS frame replaces any existing value for
that parameter. Parameters are processed in the order in which they
appear, and a receiver of a SETTINGS frame does not need to maintain
any state other than the current value of its parameters. Therefore,
the value of a SETTINGS parameter is the last value that is seen by a
receiver.
SETTINGS parameters are acknowledged by the receiving peer. To
enable this, the SETTINGS frame defines the following flag:
ACK (0x1): Bit 1 being set indicates that this frame acknowledges
receipt and application of the peer's SETTINGS frame. When this
bit is set, the payload of the SETTINGS frame MUST be empty.
Receipt of a SETTINGS frame with the ACK flag set and a length
field value other than 0 MUST be treated as a connection error
(Section 5.4.1) of type FRAME_SIZE_ERROR. For more info, see
Settings Synchronization (Section 6.5.3).
SETTINGS frames always apply to a connection, never a single stream.
The stream identifier for a SETTINGS frame MUST be zero (0x0). If an
endpoint receives a SETTINGS frame whose stream identifier field is
anything other than 0x0, the endpoint MUST respond with a connection
error (Section 5.4.1) of type PROTOCOL_ERROR.
The SETTINGS frame affects connection state. A badly formed or
incomplete SETTINGS frame MUST be treated as a connection error
(Section 5.4.1) of type PROTOCOL_ERROR.
6.5.1. SETTINGS Format
The payload of a SETTINGS frame consists of zero or more parameters,
each consisting of an unsigned 16-bit setting identifier and an
unsigned 32-bit value.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identifier (16) |
+-------------------------------+-------------------------------+
| Value (32) |
+---------------------------------------------------------------+
Setting Format
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The following parameters are defined:
SETTINGS_HEADER_TABLE_SIZE (0x1): Allows the sender to inform the
remote endpoint of the maximum size of the header compression
table used to decode header blocks. The encoder can select any
size equal to or less than this value by using signaling specific
to the header compression format inside a header block. The
initial value is 4,096 bytes.
SETTINGS_ENABLE_PUSH (0x2): This setting can be use to disable
server push (Section 8.2). An endpoint MUST NOT send a
PUSH_PROMISE frame if it receives this parameter set to a value of
0. An endpoint that has both set this parameter to 0 and had it
acknowledged MUST treat the receipt of a PUSH_PROMISE frame as a
connection error (Section 5.4.1) of type PROTOCOL_ERROR.
The initial value is 1, which indicates that server push is
permitted. Any value other than 0 or 1 MUST be treated as a
connection error (Section 5.4.1) of type PROTOCOL_ERROR.
SETTINGS_MAX_CONCURRENT_STREAMS (0x3): Indicates the maximum number
of concurrent streams that the sender will allow. This limit is
directional: it applies to the number of streams that the sender
permits the receiver to create. Initially there is no limit to
this value. It is recommended that this value be no smaller than
100, so as to not unnecessarily limit parallelism.
A value of 0 for SETTINGS_MAX_CONCURRENT_STREAMS SHOULD NOT be
treated as special by endpoints. A zero value does prevent the
creation of new streams, however this can also happen for any
limit that is exhausted with active streams. Servers SHOULD only
set a zero value for short durations; if a server does not wish to
accept requests, closing the connection could be preferable.
SETTINGS_INITIAL_WINDOW_SIZE (0x4): Indicates the sender's initial
window size (in bytes) for stream level flow control. The initial
value is 2^16-1 (65,535) octets.
This setting affects the window size of all streams, including
existing streams, see Section 6.9.2.
Values above the maximum flow control window size of 2^31-1 MUST
be treated as a connection error (Section 5.4.1) of type
FLOW_CONTROL_ERROR.
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SETTINGS_MAX_FRAME_SIZE (0x5): Indicates the size of the largest
frame payload that a receiver is willing to accept.
The initial value is 2^14 (16,384) octets. The value advertised
by an endpoint MUST be between this initial value and the maximum
allowed frame size (2^24-1 or 16,777,215 octets), inclusive.
Values outside this range MUST be treated as a connection error
(Section 5.4.1) of type PROTOCOL_ERROR.
SETTINGS_MAX_HEADER_LIST_SIZE (0x6): This advisory setting informs a
peer of the maximum size of header list that the sender is
prepared to accept. The value is based on the uncompressed size
of header fields, including the length of the name and value in
octets plus an overhead of 32 octets for each header field.
For any given request, a lower limit than what is advertised MAY
be enforced. The initial value of this setting is unlimited.
An endpoint that receives a SETTINGS frame with any unknown or
unsupported identifier MUST ignore that setting.
6.5.3. Settings Synchronization
Most values in SETTINGS benefit from or require an understanding of
when the peer has received and applied the changed parameter values.
In order to provide such synchronization timepoints, the recipient of
a SETTINGS frame in which the ACK flag is not set MUST apply the
updated parameters as soon as possible upon receipt.
The values in the SETTINGS frame MUST be processed in the order they
appear, with no other frame processing between values. Unsupported
parameters MUST be ignored. Once all values have been processed, the
recipient MUST immediately emit a SETTINGS frame with the ACK flag
set. Upon receiving a SETTINGS frame with the ACK flag set, the
sender of the altered parameters can rely on the setting having been
applied.
If the sender of a SETTINGS frame does not receive an acknowledgement
within a reasonable amount of time, it MAY issue a connection error
(Section 5.4.1) of type SETTINGS_TIMEOUT.
6.6. PUSH_PROMISE
The PUSH_PROMISE frame (type=0x5) is used to notify the peer endpoint
in advance of streams the sender intends to initiate. The
PUSH_PROMISE frame includes the unsigned 31-bit identifier of the
stream the endpoint plans to create along with a set of headers that
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provide additional context for the stream. Section 8.2 contains a
thorough description of the use of PUSH_PROMISE frames.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Pad Length? (8)|
+-+-------------+-----------------------------------------------+
|R| Promised Stream ID (31) |
+-+-----------------------------+-------------------------------+
| Header Block Fragment (*) ...
+---------------------------------------------------------------+
| Padding (*) ...
+---------------------------------------------------------------+
PUSH_PROMISE Payload Format
The PUSH_PROMISE frame payload has the following fields:
Pad Length: An 8-bit field containing the length of the frame
padding in units of octets. This field is optional and is only
present if the PADDED flag is set.
R: A single reserved bit.
Promised Stream ID: This unsigned 31-bit integer identifies the
stream the endpoint intends to start sending frames for. The
promised stream identifier MUST be a valid choice for the next
stream sent by the sender (see new stream identifier
(Section 5.1.1)).
Header Block Fragment: A header block fragment (Section 4.3)
containing request header fields.
Padding: Padding octets.
The PUSH_PROMISE frame defines the following flags:
END_HEADERS (0x4): Bit 3 being set indicates that this frame
contains an entire header block (Section 4.3) and is not followed
by any CONTINUATION frames.
A PUSH_PROMISE frame without the END_HEADERS flag set MUST be
followed by a CONTINUATION frame for the same stream. A receiver
MUST treat the receipt of any other type of frame or a frame on a
different stream as a connection error (Section 5.4.1) of type
PROTOCOL_ERROR.
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PADDED (0x8): Bit 4 being set indicates that the Pad Length field is
present.
PUSH_PROMISE frames MUST be associated with an existing, peer-
initiated stream. The stream identifier of a PUSH_PROMISE frame
indicates the stream it is associated with. If the stream identifier
field specifies the value 0x0, a recipient MUST respond with a
connection error (Section 5.4.1) of type PROTOCOL_ERROR.
Promised streams are not required to be used in the order they are
promised. The PUSH_PROMISE only reserves stream identifiers for
later use.
PUSH_PROMISE MUST NOT be sent if the SETTINGS_ENABLE_PUSH setting of
the peer endpoint is set to 0. An endpoint that has set this setting
and has received acknowledgement MUST treat the receipt of a
PUSH_PROMISE frame as a connection error (Section 5.4.1) of type
PROTOCOL_ERROR.
Recipients of PUSH_PROMISE frames can choose to reject promised
streams by returning a RST_STREAM referencing the promised stream
identifier back to the sender of the PUSH_PROMISE.
A PUSH_PROMISE frame modifies the connection state in two ways. The
inclusion of a header block (Section 4.3) potentially modifies the
state maintained for header compression. PUSH_PROMISE also reserves
a stream for later use, causing the promised stream to enter the
"reserved" state. A sender MUST NOT send a PUSH_PROMISE on a stream
unless that stream is either "open" or "half closed (remote)"; the
sender MUST ensure that the promised stream is a valid choice for a
new stream identifier (Section 5.1.1) (that is, the promised stream
MUST be in the "idle" state).
Since PUSH_PROMISE reserves a stream, ignoring a PUSH_PROMISE frame
causes the stream state to become indeterminate. A receiver MUST
treat the receipt of a PUSH_PROMISE on a stream that is neither
"open" nor "half closed (local)" as a connection error
(Section 5.4.1) of type PROTOCOL_ERROR. Similarly, a receiver MUST
treat the receipt of a PUSH_PROMISE that promises an illegal stream
identifier (Section 5.1.1) (that is, an identifier for a stream that
is not currently in the "idle" state) as a connection error
(Section 5.4.1) of type PROTOCOL_ERROR.
The PUSH_PROMISE frame includes optional padding. Padding fields and
flags are identical to those defined for DATA frames (Section 6.1).
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The PING frame (type=0x6) is a mechanism for measuring a minimal
round trip time from the sender, as well as determining whether an
idle connection is still functional. PING frames can be sent from
any endpoint.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Opaque Data (64) |
| |
+---------------------------------------------------------------+
PING Payload Format
In addition to the frame header, PING frames MUST contain 8 octets of
data in the payload. A sender can include any value it chooses and
use those bytes in any fashion.
Receivers of a PING frame that does not include an ACK flag MUST send
a PING frame with the ACK flag set in response, with an identical
payload. PING responses SHOULD be given higher priority than any
other frame.
The PING frame defines the following flags:
ACK (0x1): Bit 1 being set indicates that this PING frame is a PING
response. An endpoint MUST set this flag in PING responses. An
endpoint MUST NOT respond to PING frames containing this flag.
PING frames are not associated with any individual stream. If a PING
frame is received with a stream identifier field value other than
0x0, the recipient MUST respond with a connection error
(Section 5.4.1) of type PROTOCOL_ERROR.
Receipt of a PING frame with a length field value other than 8 MUST
be treated as a connection error (Section 5.4.1) of type
FRAME_SIZE_ERROR.
6.8. GOAWAY
The GOAWAY frame (type=0x7) informs the remote peer to stop creating
streams on this connection. GOAWAY can be sent by either the client
or the server. Once sent, the sender will ignore frames sent on any
new streams with identifiers higher than the included last stream
identifier. Receivers of a GOAWAY frame MUST NOT open additional
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streams on the connection, although a new connection can be
established for new streams.
The purpose of this frame is to allow an endpoint to gracefully stop
accepting new streams, while still finishing processing of previously
established streams. This enables administrative actions, like
server maintainence.
There is an inherent race condition between an endpoint starting new
streams and the remote sending a GOAWAY frame. To deal with this
case, the GOAWAY contains the stream identifier of the last peer-
initiated stream which was or might be processed on the sending
endpoint in this connection. For instance, if the server sends a
GOAWAY frame, the identifed stream is the highest numbered stream
initiated by the client.
If the receiver of the GOAWAY has sent data on streams with a higher
stream identifier than what is indicated in the GOAWAY frame, those
streams are not or will not be processed. The receiver of the GOAWAY
frame can treat the streams as though they had never been created at
all, thereby allowing those streams to be retried later on a new
connection.
Endpoints SHOULD always send a GOAWAY frame before closing a
connection so that the remote can know whether a stream has been
partially processed or not. For example, if an HTTP client sends a
POST at the same time that a server closes a connection, the client
cannot know if the server started to process that POST request if the
server does not send a GOAWAY frame to indicate what streams it might
have acted on.
An endpoint might choose to close a connection without sending GOAWAY
for misbehaving peers.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|R| Last-Stream-ID (31) |
+-+-------------------------------------------------------------+
| Error Code (32) |
+---------------------------------------------------------------+
| Additional Debug Data (*) |
+---------------------------------------------------------------+
GOAWAY Payload Format
The GOAWAY frame does not define any flags.
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The GOAWAY frame applies to the connection, not a specific stream.
An endpoint MUST treat a GOAWAY frame with a stream identifier other
than 0x0 as a connection error (Section 5.4.1) of type
PROTOCOL_ERROR.
The last stream identifier in the GOAWAY frame contains the highest
numbered stream identifier for which the sender of the GOAWAY frame
might have taken some action on, or might yet take action on. All
streams up to and including the identified stream might have been
processed in some way. The last stream identifier can be set to 0 if
no streams were processed.
Note: In this context, "processed" means that some data from the
stream was passed to some higher layer of software that might have
taken some action as a result.
If a connection terminates without a GOAWAY frame, the last stream
identifier is effectively the highest possible stream identifier.
On streams with lower or equal numbered identifiers that were not
closed completely prior to the connection being closed, re-attempting
requests, transactions, or any protocol activity is not possible,
with the exception of idempotent actions like HTTP GET, PUT, or
DELETE. Any protocol activity that uses higher numbered streams can
be safely retried using a new connection.
Activity on streams numbered lower or equal to the last stream
identifier might still complete successfully. The sender of a GOAWAY
frame might gracefully shut down a connection by sending a GOAWAY
frame, maintaining the connection in an open state until all in-
progress streams complete.
An endpoint MAY send multiple GOAWAY frames if circumstances change.
For instance, an endpoint that sends GOAWAY with NO_ERROR during
graceful shutdown could subsequently encounter an condition that
requires immediate termination of the connection. The last stream
identifier from the last GOAWAY frame received indicates which
streams could have been acted upon. Endpoints MUST NOT increase the
value they send in the last stream identifier, since the peers might
already have retried unprocessed requests on another connection.
A client that is unable to retry requests loses all requests that are
in flight when the server closes the connection. This is especially
true for intermediaries that might not be serving clients using
HTTP/2. A server that is attempting to gracefully shut down a
connection SHOULD send an initial GOAWAY frame with the last stream
identifier set to 2^31-1 and a NO_ERROR code. This signals to the
client that a shutdown is imminent and that no further requests can
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be initiated. After waiting at least one round trip time, the server
can send another GOAWAY frame with an updated last stream identifier.
This ensures that a connection can be cleanly shut down without
losing requests.
After sending a GOAWAY frame, the sender can discard frames for
streams with identifiers higher than the identified last stream.
However, any frames that alter connection state cannot be completely
ignored. For instance, HEADERS, PUSH_PROMISE and CONTINUATION frames
MUST be minimally processed to ensure the state maintained for header
compression is consistent (see Section 4.3); similarly DATA frames
MUST be counted toward the connection flow control window. Failure
to process these frames can cause flow control or header compression
state to become unsynchronized.
The GOAWAY frame also contains a 32-bit error code (Section 7) that
contains the reason for closing the connection.
Endpoints MAY append opaque data to the payload of any GOAWAY frame.
Additional debug data is intended for diagnostic purposes only and
carries no semantic value. Debug information could contain security-
or privacy-sensitive data. Logged or otherwise persistently stored
debug data MUST have adequate safeguards to prevent unauthorized
access.
6.9. WINDOW_UPDATE
The WINDOW_UPDATE frame (type=0x8) is used to implement flow control;
see Section 5.2 for an overview.
Flow control operates at two levels: on each individual stream and on
the entire connection.
Both types of flow control are hop-by-hop; that is, only between the
two endpoints. Intermediaries do not forward WINDOW_UPDATE frames
between dependent connections. However, throttling of data transfer
by any receiver can indirectly cause the propagation of flow control
information toward the original sender.
Flow control only applies to frames that are identified as being
subject to flow control. Of the frame types defined in this
document, this includes only DATA frames. Frames that are exempt
from flow control MUST be accepted and processed, unless the receiver
is unable to assign resources to handling the frame. A receiver MAY
respond with a stream error (Section 5.4.2) or connection error
(Section 5.4.1) of type FLOW_CONTROL_ERROR if it is unable to accept
a frame.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|R| Window Size Increment (31) |
+-+-------------------------------------------------------------+
WINDOW_UPDATE Payload Format
The payload of a WINDOW_UPDATE frame is one reserved bit, plus an
unsigned 31-bit integer indicating the number of bytes that the
sender can transmit in addition to the existing flow control window.
The legal range for the increment to the flow control window is 1 to
2^31-1 (0x7fffffff) bytes.
The WINDOW_UPDATE frame does not define any flags.
The WINDOW_UPDATE frame can be specific to a stream or to the entire
connection. In the former case, the frame's stream identifier
indicates the affected stream; in the latter, the value "0" indicates
that the entire connection is the subject of the frame.
A receiver MUST treat the recipt of a WINDOW_UPDATE frame with an
flow control window increment of 0 as a stream error (Section 5.4.2)
of type PROTOCOL_ERROR; errors on the connection flow control window
MUST be treated as a connection error (Section 5.4.1).
WINDOW_UPDATE can be sent by a peer that has sent a frame bearing the
END_STREAM flag. This means that a receiver could receive a
WINDOW_UPDATE frame on a "half closed (remote)" or "closed" stream.
A receiver MUST NOT treat this as an error, see Section 5.1.
A receiver that receives a flow controlled frame MUST always account
for its contribution against the connection flow control window,
unless the receiver treats this as a connection error
(Section 5.4.1). This is necessary even if the frame is in error.
Since the sender counts the frame toward the flow control window, if
the receiver does not, the flow control window at sender and receiver
can become different.
6.9.1. The Flow Control Window
Flow control in HTTP/2 is implemented using a window kept by each
sender on every stream. The flow control window is a simple integer
value that indicates how many bytes of data the sender is permitted
to transmit; as such, its size is a measure of the buffering capacity
of the receiver.
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Two flow control windows are applicable: the stream flow control
window and the connection flow control window. The sender MUST NOT
send a flow controlled frame with a length that exceeds the space
available in either of the flow control windows advertised by the
receiver. Frames with zero length with the END_STREAM flag set (that
is, an empty DATA frame) MAY be sent if there is no available space
in either flow control window.
For flow control calculations, the 8 byte frame header is not
counted.
After sending a flow controlled frame, the sender reduces the space
available in both windows by the length of the transmitted frame.
The receiver of a frame sends a WINDOW_UPDATE frame as it consumes
data and frees up space in flow control windows. Separate
WINDOW_UPDATE frames are sent for the stream and connection level
flow control windows.
A sender that receives a WINDOW_UPDATE frame updates the
corresponding window by the amount specified in the frame.
A sender MUST NOT allow a flow control window to exceed 2^31-1 bytes.
If a sender receives a WINDOW_UPDATE that causes a flow control
window to exceed this maximum it MUST terminate either the stream or
the connection, as appropriate. For streams, the sender sends a
RST_STREAM with the error code of FLOW_CONTROL_ERROR code; for the
connection, a GOAWAY frame with a FLOW_CONTROL_ERROR code.
Flow controlled frames from the sender and WINDOW_UPDATE frames from
the receiver are completely asynchronous with respect to each other.
This property allows a receiver to aggressively update the window
size kept by the sender to prevent streams from stalling.
6.9.2. Initial Flow Control Window Size
When an HTTP/2 connection is first established, new streams are
created with an initial flow control window size of 65,535 bytes.
The connection flow control window is 65,535 bytes. Both endpoints
can adjust the initial window size for new streams by including a
value for SETTINGS_INITIAL_WINDOW_SIZE in the SETTINGS frame that
forms part of the connection preface. The connection flow control
window can only be changed using WINDOW_UPDATE frames.
Prior to receiving a SETTINGS frame that sets a value for
SETTINGS_INITIAL_WINDOW_SIZE, an endpoint can only use the default
initial window size when sending flow controlled frames. Similarly,
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the connection flow control window is set to the default initial
window size until a WINDOW_UPDATE frame is received.
A SETTINGS frame can alter the initial flow control window size for
all current streams. When the value of SETTINGS_INITIAL_WINDOW_SIZE
changes, a receiver MUST adjust the size of all stream flow control
windows that it maintains by the difference between the new value and
the old value.
A change to SETTINGS_INITIAL_WINDOW_SIZE can cause the available
space in a flow control window to become negative. A sender MUST
track the negative flow control window, and MUST NOT send new flow
controlled frames until it receives WINDOW_UPDATE frames that cause
the flow control window to become positive.
For example, if the client sends 60KB immediately on connection
establishment, and the server sets the initial window size to be
16KB, the client will recalculate the available flow control window
to be -44KB on receipt of the SETTINGS frame. The client retains a
negative flow control window until WINDOW_UPDATE frames restore the
window to being positive, after which the client can resume sending.
A SETTINGS frame cannot alter the connection flow control window.
An endpoint MUST treat a change to SETTINGS_INITIAL_WINDOW_SIZE that
causes any flow control window to exceed the maximum size as a
connection error (Section 5.4.1) of type FLOW_CONTROL_ERROR.
6.9.3. Reducing the Stream Window Size
A receiver that wishes to use a smaller flow control window than the
current size can send a new SETTINGS frame. However, the receiver
MUST be prepared to receive data that exceeds this window size, since
the sender might send data that exceeds the lower limit prior to
processing the SETTINGS frame.
After sending a SETTINGS frame that reduces the initial flow control
window size, a receiver has two options for handling streams that
exceed flow control limits:
1. The receiver can immediately send RST_STREAM with
FLOW_CONTROL_ERROR error code for the affected streams.
2. The receiver can accept the streams and tolerate the resulting
head of line blocking, sending WINDOW_UPDATE frames as it
consumes data.
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The CONTINUATION frame (type=0x9) is used to continue a sequence of
header block fragments (Section 4.3). Any number of CONTINUATION
frames can be sent on an existing stream, as long as the preceding
frame is on the same stream and is a HEADERS, PUSH_PROMISE or
CONTINUATION frame without the END_HEADERS flag set.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Header Block Fragment (*) ...
+---------------------------------------------------------------+
CONTINUATION Frame Payload
The CONTINUATION frame payload contains a header block fragment
(Section 4.3).
The CONTINUATION frame defines the following flag:
END_HEADERS (0x4): Bit 3 being set indicates that this frame ends a
header block (Section 4.3).
If the END_HEADERS bit is not set, this frame MUST be followed by
another CONTINUATION frame. A receiver MUST treat the receipt of
any other type of frame or a frame on a different stream as a
connection error (Section 5.4.1) of type PROTOCOL_ERROR.
The CONTINUATION frame changes the connection state as defined in
Section 4.3.
CONTINUATION frames MUST be associated with a stream. If a
CONTINUATION frame is received whose stream identifier field is 0x0,
the recipient MUST respond with a connection error (Section 5.4.1) of
type PROTOCOL_ERROR.
A CONTINUATION frame MUST be preceded by a HEADERS, PUSH_PROMISE or
CONTINUATION frame without the END_HEADERS flag set. A recipient
that observes violation of this rule MUST respond with a connection
error (Section 5.4.1) of type PROTOCOL_ERROR.
7. Error Codes
Error codes are 32-bit fields that are used in RST_STREAM and GOAWAY
frames to convey the reasons for the stream or connection error.
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Error codes share a common code space. Some error codes apply only
to either streams or the entire connection and have no defined
semantics in the other context.
The following error codes are defined:
NO_ERROR (0x0): The associated condition is not as a result of an
error. For example, a GOAWAY might include this code to indicate
graceful shutdown of a connection.
PROTOCOL_ERROR (0x1): The endpoint detected an unspecific protocol
error. This error is for use when a more specific error code is
not available.
INTERNAL_ERROR (0x2): The endpoint encountered an unexpected
internal error.
FLOW_CONTROL_ERROR (0x3): The endpoint detected that its peer
violated the flow control protocol.
SETTINGS_TIMEOUT (0x4): The endpoint sent a SETTINGS frame, but did
not receive a response in a timely manner. See Settings
Synchronization (Section 6.5.3).
STREAM_CLOSED (0x5): The endpoint received a frame after a stream
was half closed.
FRAME_SIZE_ERROR (0x6): The endpoint received a frame that was
larger than the maximum size that it supports.
REFUSED_STREAM (0x7): The endpoint refuses the stream prior to
performing any application processing, see Section 8.1.4 for
details.
CANCEL (0x8): Used by the endpoint to indicate that the stream is no
longer needed.
COMPRESSION_ERROR (0x9): The endpoint is unable to maintain the
header compression context for the connection.
CONNECT_ERROR (0xa): The connection established in response to a
CONNECT request (Section 8.3) was reset or abnormally closed.
ENHANCE_YOUR_CALM (0xb): The endpoint detected that its peer is
exhibiting a behavior that might be generating excessive load.
INADEQUATE_SECURITY (0xc): The underlying transport has properties
that do not meet minimum security requirements (see Section 9.2).
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Unknown or unsupported error codes MUST NOT trigger any special
behavior. These MAY be treated by an implementation as being
equivalent to INTERNAL_ERROR.
8. HTTP Message Exchanges
HTTP/2 is intended to be as compatible as possible with current uses
of HTTP. This means that, from the application perspective, the
features of the protocol are largely unchanged. To achieve this, all
request and response semantics are preserved, although the syntax of
conveying those semantics has changed.
Thus, the specification and requirements of HTTP/1.1 Semantics and
Content [RFC7231], Conditional Requests [RFC7232], Range Requests
[RFC7233], Caching [RFC7234] and Authentication [RFC7235] are
applicable to HTTP/2. Selected portions of HTTP/1.1 Message Syntax
and Routing [RFC7230], such as the HTTP and HTTPS URI schemes, are
also applicable in HTTP/2, but the expression of those semantics for
this protocol are defined in the sections below.
8.1. HTTP Request/Response Exchange
A client sends an HTTP request on a new stream, using a previously
unused stream identifier (Section 5.1.1). A server sends an HTTP
response on the same stream as the request.
An HTTP message (request or response) consists of:
1. for a response only, zero or more HEADERS frames (each followed
by zero or more CONTINUATION frames) containing the message
headers of informational (1xx) HTTP responses (see [RFC7230],
Section 3.2 and [RFC7231], Section 6.2), and
2. one HEADERS frame (followed by zero or more CONTINUATION frames)
containing the message headers (see [RFC7230], Section 3.2), and
3. zero or more DATA frames containing the message payload (see
[RFC7230], Section 3.3), and
4. optionally, one HEADERS frame, followed by zero or more
CONTINUATION frames containing the trailer-part, if present (see
[RFC7230], Section 4.1.2).
The last frame in the sequence bears an END_STREAM flag, noting that
a HEADERS frame bearing the END_STREAM flag can be followed by
CONTINUATION frames that carry any remaining portions of the header
block.
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Other frames (from any stream) MUST NOT occur between either HEADERS
frame and any CONTINUATION frames that might follow.
A HEADERS frame (and associated CONTINUATION frames) can only appear
at the start or end of a stream. An endpoint that receives a second
HEADERS frame without the END_STREAM flag set MUST treat the
corresponding request or response as malformed (Section 8.1.2.6).
Trailing header fields are carried in a header block that also
terminates the stream. That is, a sequence starting with a HEADERS
frame, followed by zero or more CONTINUATION frames, where the
HEADERS frame bears an END_STREAM flag. Header blocks after the
first that do not terminate the stream are not part of an HTTP
request or response.
An HTTP request/response exchange fully consumes a single stream. A
request starts with the HEADERS frame that puts the stream into an
"open" state. The request ends with a frame bearing END_STREAM,
which causes the stream to become "half closed (local)" for the
client and "half closed (remote)" for the server. A response starts
with a HEADERS frame and ends with a frame bearing END_STREAM, which
places the stream in the "closed" state.
8.1.1. Upgrading From HTTP/2
HTTP/2 removes support for the 101 (Switching Protocols)
informational status code ([RFC7231], Section 6.2.2).
The semantics of 101 (Switching Protocols) aren't applicable to a
multiplexed protocol. Alternative protocols are able to use the same
mechanisms that HTTP/2 uses to negotiate their use (see Section 3).
8.1.2. HTTP Header Fields
HTTP header fields carry information as a series of key-value pairs.
For a listing of registered HTTP headers, see the Message Header
Field Registry maintained at [4].
8.1.2.1. Pseudo-Header Fields
While HTTP/1.x used the message start-line (see [RFC7230],
Section 3.1) to convey the target URI and method of the request, and
the status code for the response, HTTP/2 uses special pseudo-header
fields beginning with ':' character (ASCII 0x3a) for this purpose.
Pseudo-header fields are only valid in the HTTP/2 context. These are
not HTTP header fields. Endpoints MUST NOT generate pseudo-header
fields other than those defined in this document.
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Pseudo-header fields are only valid in the context in which they are
defined. Pseudo-header fields defined for requests MUST NOT appear
in responses; pseudo-header fields defined for responses MUST NOT
appear in requests. Pseudo-header fields MUST NOT appear in
trailers. Endpoints MUST treat a request or response that contains
undefined or invalid pseudo-header fields as malformed
(Section 8.1.2.6).
Just as in HTTP/1.x, header field names are strings of ASCII
characters that are compared in a case-insensitive fashion. However,
header field names MUST be converted to lowercase prior to their
encoding in HTTP/2. A request or response containing uppercase
header field names MUST be treated as malformed (Section 8.1.2.6).
All pseudo-header fields MUST appear in the header block before
regular header fields. Any request or response that contains a
pseudo-header field that appears in a header block after a regular
header field MUST be treated as malformed (Section 8.1.2.6).
8.1.2.2. Hop-by-Hop Header Fields
HTTP/2 does not use the Connection header field to indicate "hop-by-
hop" header fields; in this protocol, connection-specific metadata is
conveyed by other means. As such, a HTTP/2 message containing
Connection MUST be treated as malformed (Section 8.1.2.6).
This means that an intermediary transforming an HTTP/1.x message to
HTTP/2 will need to remove any header fields nominated by the
Connection header field, along with the Connection header field
itself. Such intermediaries SHOULD also remove other connection-
specific header fields, such as Keep-Alive, Proxy-Connection,
Transfer-Encoding and Upgrade, even if they are not nominated by
Connection.
One exception to this is the TE header field, which MAY be present in
an HTTP/2 request, but when it is MUST NOT contain any value other
than "trailers".
Note: HTTP/2 purposefully does not support upgrade to another
protocol. The handshake methods described in Section 3 are
believed sufficient to negotiate the use of alternative protocols.
8.1.2.3. Request Header Fields
HTTP/2 defines a number of pseudo header fields starting with a colon
':' character that carry information about the request target:
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o The ":method" header field includes the HTTP method ([RFC7231],
Section 4).
o The ":scheme" header field includes the scheme portion of the
target URI ([RFC3986], Section 3.1).
":scheme" is not restricted to "http" and "https" schemed URIs. A
proxy or gateway can translate requests for non-HTTP schemes,
enabling the use of HTTP to interact with non-HTTP services.
o The ":authority" header field includes the authority portion of
the target URI ([RFC3986], Section 3.2). The authority MUST NOT
include the deprecated "userinfo" subcomponent for "http" or
"https" schemed URIs.
To ensure that the HTTP/1.1 request line can be reproduced
accurately, this header field MUST be omitted when translating
from an HTTP/1.1 request that has a request target in origin or
asterisk form (see [RFC7230], Section 5.3). Clients that generate
HTTP/2 requests directly SHOULD instead omit the "Host" header
field. An intermediary that converts an HTTP/2 request to
HTTP/1.1 MUST create a "Host" header field if one is not present
in a request by copying the value of the ":authority" header
field.
o The ":path" header field includes the path and query parts of the
target URI (the "path-absolute" production from [RFC3986] and
optionally a '?' character followed by the "query" production, see
[RFC3986], Section 3.3 and [RFC3986], Section 3.4). A request in
asterisk form includes the value '*' for the ":path" header field.
This field MUST NOT be empty for "http" or "https" URIs; "http" or
"https" URIs that do not contain a path component MUST include a
value of '/'. The exception to this rule is an OPTIONS request
for an "http" or "https" URI that does not include a path
component; these MUST include a ":path" header field with a value
of '*' (see [RFC7230], Section 5.3.4).
All HTTP/2 requests MUST include exactly one valid value for the
":method", ":scheme", and ":path" header fields, unless this is a
CONNECT request (Section 8.3). An HTTP request that omits mandatory
header fields is malformed (Section 8.1.2.6).
HTTP/2 does not define a way to carry the version identifier that is
included in the HTTP/1.1 request line.
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A single ":status" header field is defined that carries the HTTP
status code field (see [RFC7231], Section 6). This header field MUST
be included in all responses, otherwise the response is malformed
(Section 8.1.2.6).
HTTP/2 does not define a way to carry the version or reason phrase
that is included in an HTTP/1.1 status line.
8.1.2.5. Compressing the Cookie Header Field
The Cookie header field [COOKIE] can carry a significant amount of
redundant data.
The Cookie header field uses a semi-colon (";") to delimit cookie-
pairs (or "crumbs"). This header field doesn't follow the list
construction rules in HTTP (see [RFC7230], Section 3.2.2), which
prevents cookie-pairs from being separated into different name-value
pairs. This can significantly reduce compression efficiency as
individual cookie-pairs are updated.
To allow for better compression efficiency, the Cookie header field
MAY be split into separate header fields, each with one or more
cookie-pairs. If there are multiple Cookie header fields after
decompression, these MUST be concatenated into a single octet string
using the two octet delimiter of 0x3B, 0x20 (the ASCII string "; ")
before being passed into a non-HTTP/2 context, such as an HTTP/1.1
connection, or a generic HTTP server application.
Therefore, the following two lists of Cookie header fields are
semantically equivalent.
cookie: a=b; c=d; e=f
cookie: a=b
cookie: c=d
cookie: e=f
8.1.2.6. Malformed Requests and Responses
A malformed request or response is one that is an otherwise valid
sequence of HTTP/2 frames, but is otherwise invalid due to the
presence of extraneous frames, prohibited header fields, the absence
of mandatory header fields, or the inclusion of uppercase header
field names.
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A request or response that includes an entity body can include a
"content-length" header field. A request or response is also
malformed if the value of a "content-length" header field does not
equal the sum of the DATA frame payload lengths that form the body,
with the exception of responses to HEAD requests, which always
contain no DATA frames.
Intermediaries that process HTTP requests or responses (i.e., any
intermediary not acting as a tunnel) MUST NOT forward a malformed
request or response. Malformed requests or responses that are
detected MUST be treated as a stream error (Section 5.4.2) of type
PROTOCOL_ERROR.
For malformed requests, a server MAY send an HTTP response prior to
closing or resetting the stream. Clients MUST NOT accept a malformed
response. Note that these requirements are intended to protect
against several types of common attacks against HTTP; they are
deliberately strict, because being permissive can expose
implementations to these vulnerabilities.
8.1.3. Examples
This section shows HTTP/1.1 requests and responses, with
illustrations of equivalent HTTP/2 requests and responses.
An HTTP GET request includes request header fields and no body and is
therefore transmitted as a single HEADERS frame, followed by zero or
more CONTINUATION frames containing the serialized block of request
header fields. The HEADERS frame in the following has both the
END_HEADERS and END_STREAM flags set; no CONTINUATION frames are
sent:
GET /resource HTTP/1.1 HEADERS
Host: example.org ==> + END_STREAM
Accept: image/jpeg + END_HEADERS
:method = GET
:scheme = https
:path = /resource
host = example.org
accept = image/jpeg
Similarly, a response that includes only response header fields is
transmitted as a HEADERS frame (again, followed by zero or more
CONTINUATION frames) containing the serialized block of response
header fields.
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HTTP/1.1 304 Not Modified HEADERS
ETag: "xyzzy" ==> + END_STREAM
Expires: Thu, 23 Jan ... + END_HEADERS
:status = 304
etag = "xyzzy"
expires = Thu, 23 Jan ...
An HTTP POST request that includes request header fields and payload
data is transmitted as one HEADERS frame, followed by zero or more
CONTINUATION frames containing the request header fields, followed by
one or more DATA frames, with the last CONTINUATION (or HEADERS)
frame having the END_HEADERS flag set and the final DATA frame having
the END_STREAM flag set:
POST /resource HTTP/1.1 HEADERS
Host: example.org ==> - END_STREAM
Content-Type: image/jpeg - END_HEADERS
Content-Length: 123 :method = POST
:path = /resource
{binary data} content-type = image/jpeg
CONTINUATION
+ END_HEADERS
host = example.org
:scheme = https
content-length = 123
DATA
+ END_STREAM
{binary data}
Note that data contributing to any given header field could be spread
between header block fragments. The allocation of header fields to
frames in this example is illustrative only.
A response that includes header fields and payload data is
transmitted as a HEADERS frame, followed by zero or more CONTINUATION
frames, followed by one or more DATA frames, with the last DATA frame
in the sequence having the END_STREAM flag set:
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occurred prior to the error, which could result in undesirable
effects if the request were reattempted.
HTTP/2 provides two mechanisms for providing a guarantee to a client
that a request has not been processed:
o The GOAWAY frame indicates the highest stream number that might
have been processed. Requests on streams with higher numbers are
therefore guaranteed to be safe to retry.
o The REFUSED_STREAM error code can be included in a RST_STREAM
frame to indicate that the stream is being closed prior to any
processing having occurred. Any request that was sent on the
reset stream can be safely retried.
Requests that have not been processed have not failed; clients MAY
automatically retry them, even those with non-idempotent methods.
A server MUST NOT indicate that a stream has not been processed
unless it can guarantee that fact. If frames that are on a stream
are passed to the application layer for any stream, then
REFUSED_STREAM MUST NOT be used for that stream, and a GOAWAY frame
MUST include a stream identifier that is greater than or equal to the
given stream identifier.
In addition to these mechanisms, the PING frame provides a way for a
client to easily test a connection. Connections that remain idle can
become broken as some middleboxes (for instance, network address
translators, or load balancers) silently discard connection bindings.
The PING frame allows a client to safely test whether a connection is
still active without sending a request.
8.2. Server Push
HTTP/2 enables a server to pre-emptively send (or "push") one or more
associated responses to a client in response to a single request.
This feature becomes particularly helpful when the server knows the
client will need to have those responses available in order to fully
process the response to the original request.
Pushing additional responses is optional, and is negotiated between
individual endpoints. The SETTINGS_ENABLE_PUSH setting can be set to
0 to indicate that server push is disabled.
Because pushing responses is effectively hop-by-hop, an intermediary
could receive pushed responses from the server and choose not to
forward those on to the client. In other words, how to make use of
the pushed responses is up to that intermediary. Equally, the
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intermediary might choose to push additional responses to the client,
without any action taken by the server.
A client cannot push. Thus, servers MUST treat the receipt of a
PUSH_PROMISE frame as a connection error (Section 5.4.1) of type
PROTOCOL_ERROR. Clients MUST reject any attempt to change the
SETTINGS_ENABLE_PUSH setting to a value other than 0 by treating the
message as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.
A server can only push responses that are cacheable (see [RFC7234],
Section 3); promised requests MUST be safe (see [RFC7231],
Section 4.2.1) and MUST NOT include a request body.
8.2.1. Push Requests
Server push is semantically equivalent to a server responding to a
request; however, in this case that request is also sent by the
server, as a PUSH_PROMISE frame.
The PUSH_PROMISE frame includes a header block that contains a
complete set of request header fields that the server attributes to
the request. It is not possible to push a response to a request that
includes a request body.
Pushed responses are always associated with an explicit request from
the client. The PUSH_PROMISE frames sent by the server are sent on
that explicit request's stream. The PUSH_PROMISE frame also includes
a promised stream identifier, chosen from the stream identifiers
available to the server (see Section 5.1.1).
The header fields in PUSH_PROMISE and any subsequent CONTINUATION
frames MUST be a valid and complete set of request header fields
(Section 8.1.2.3). The server MUST include a method in the ":method"
header field that is safe and cacheable. If a client receives a
PUSH_PROMISE that does not include a complete and valid set of header
fields, or the ":method" header field identifies a method that is not
safe, it MUST respond with a stream error (Section 5.4.2) of type
PROTOCOL_ERROR.
The server SHOULD send PUSH_PROMISE (Section 6.6) frames prior to
sending any frames that reference the promised responses. This
avoids a race where clients issue requests prior to receiving any
PUSH_PROMISE frames.
For example, if the server receives a request for a document
containing embedded links to multiple image files, and the server
chooses to push those additional images to the client, sending push
promises before the DATA frames that contain the image links ensures
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that the client is able to see the promises before discovering
embedded links. Similarly, if the server pushes responses referenced
by the header block (for instance, in Link header fields), sending
the push promises before sending the header block ensures that
clients do not request them.
PUSH_PROMISE frames MUST NOT be sent by the client.
PUSH_PROMISE frames can be sent by the server in response to any
client-initiated stream, but the stream MUST be in either the "open"
or "half closed (remote)" state with respect to the server.
PUSH_PROMISE frames are interspersed with the frames that comprise a
response, though they cannot be interspersed with HEADERS and
CONTINUATION frames that comprise a single header block.
Sending a PUSH_PROMISE frame creates a new stream and puts the stream
into the "reserved (local)" state for the server and the "reserved
(remote)" state for the client.
8.2.2. Push Responses
After sending the PUSH_PROMISE frame, the server can begin delivering
the pushed response as a response (Section 8.1.2.4) on a server-
initiated stream that uses the promised stream identifier. The
server uses this stream to transmit an HTTP response, using the same
sequence of frames as defined in Section 8.1. This stream becomes
"half closed" to the client (Section 5.1) after the initial HEADERS
frame is sent.
Once a client receives a PUSH_PROMISE frame and chooses to accept the
pushed response, the client SHOULD NOT issue any requests for the
promised response until after the promised stream has closed.
If the client determines, for any reason, that it does not wish to
receive the pushed response from the server, or if the server takes
too long to begin sending the promised response, the client can send
an RST_STREAM frame, using either the CANCEL or REFUSED_STREAM codes,
and referencing the pushed stream's identifier.
A client can use the SETTINGS_MAX_CONCURRENT_STREAMS setting to limit
the number of responses that can be concurrently pushed by a server.
Advertising a SETTINGS_MAX_CONCURRENT_STREAMS value of zero disables
server push by preventing the server from creating the necessary
streams. This does not prohibit a server from sending PUSH_PROMISE
frames; clients need to reset any promised streams that are not
wanted.
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Clients receiving a pushed response MUST validate that the server is
authorized to provide the response, see Section 10.1. For example, a
server that offers a certificate for only the "example.com" DNS-ID or
Common Name is not permitted to push a response for
"https://www.example.org/doc".
The response for a PUSH_PROMISE stream begins with a HEADERS frame,
which immediately puts the stream into the "half closed (remote)"
state for the server and "half closed (local)" state for the client,
and ends with a frame bearing END_STREAM, which places the stream in
the "closed" state.
Note: The client never sends a frame with the END_STREAM flag for a
server push.
8.3. The CONNECT Method
In HTTP/1.x, the pseudo-method CONNECT ([RFC7231], Section 4.3.6) is
used to convert an HTTP connection into a tunnel to a remote host.
CONNECT is primarily used with HTTP proxies to establish a TLS
session with an origin server for the purposes of interacting with
"https" resources.
In HTTP/2, the CONNECT method is used to establish a tunnel over a
single HTTP/2 stream to a remote host, for similar purposes. The
HTTP header field mapping works as mostly as defined in Request
Header Fields (Section 8.1.2.3), with a few differences.
Specifically:
o The ":method" header field is set to "CONNECT".
o The ":scheme" and ":path" header fields MUST be omitted.
o The ":authority" header field contains the host and port to
connect to (equivalent to the authority-form of the request-target
of CONNECT requests, see [RFC7230], Section 5.3).
A proxy that supports CONNECT establishes a TCP connection [TCP] to
the server identified in the ":authority" header field. Once this
connection is successfully established, the proxy sends a HEADERS
frame containing a 2xx series status code to the client, as defined
in [RFC7231], Section 4.3.6.
After the initial HEADERS frame sent by each peer, all subsequent
DATA frames correspond to data sent on the TCP connection. The
payload of any DATA frames sent by the client are transmitted by the
proxy to the TCP server; data received from the TCP server is
assembled into DATA frames by the proxy. Frame types other than DATA
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or stream management frames (RST_STREAM, WINDOW_UPDATE, and PRIORITY)
MUST NOT be sent on a connected stream, and MUST be treated as a
stream error (Section 5.4.2) if received.
The TCP connection can be closed by either peer. The END_STREAM flag
on a DATA frame is treated as being equivalent to the TCP FIN bit. A
client is expected to send a DATA frame with the END_STREAM flag set
after receiving a frame bearing the END_STREAM flag. A proxy that
receives a DATA frame with the END_STREAM flag set sends the attached
data with the FIN bit set on the last TCP segment. A proxy that
receives a TCP segment with the FIN bit set sends a DATA frame with
the END_STREAM flag set. Note that the final TCP segment or DATA
frame could be empty.
A TCP connection error is signaled with RST_STREAM. A proxy treats
any error in the TCP connection, which includes receiving a TCP
segment with the RST bit set, as a stream error (Section 5.4.2) of
type CONNECT_ERROR. Correspondingly, a proxy MUST send a TCP segment
with the RST bit set if it detects an error with the stream or the
HTTP/2 connection.
9. Additional HTTP Requirements/Considerations
This section outlines attributes of the HTTP protocol that improve
interoperability, reduce exposure to known security vulnerabilities,
or reduce the potential for implementation variation.
9.1. Connection Management
HTTP/2 connections are persistent. For best performance, it is
expected clients will not close connections until it is determined
that no further communication with a server is necessary (for
example, when a user navigates away from a particular web page), or
until the server closes the connection.
Clients SHOULD NOT open more than one HTTP/2 connection to a given
host and port pair, where host is derived from a URI, a selected
alternative service [ALT-SVC], or a configured proxy.
A client can create additional connections as replacements, either to
replace connections that are near to exhausting the available stream
identifier space (Section 5.1.1), to refresh the keying material for
a TLS connection, or to replace connections that have encountered
errors (Section 5.4.1).
A client MAY open multiple connections to the same IP address and TCP
port using different Server Name Indication [TLS-EXT] values or to
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provide different TLS client certificates, but SHOULD avoid creating
multiple connections with the same configuration.
Servers are encouraged to maintain open connections for as long as
possible, but are permitted to terminate idle connections if
necessary. When either endpoint chooses to close the transport-level
TCP connection, the terminating endpoint SHOULD first send a GOAWAY
(Section 6.8) frame so that both endpoints can reliably determine
whether previously sent frames have been processed and gracefully
complete or terminate any necessary remaining tasks.
9.1.1. Connection Reuse
Clients MAY use a single server connection to send requests for URIs
with multiple different authority components as long as the server is
authoritative (Section 10.1). For "http" resources, this depends on
the host having resolved to the same IP address.
For "https" resources, connection reuse additionally depends on
having a certificate that is valid for the host in the URI. That is
the use of server certificate with multiple "subjectAltName"
attributes, or names with wildcards. For example, a certificate with
a "subjectAltName" of "*.example.com" might permit the use of the
same connection for "a.example.com" and "b.example.com".
In some deployments, reusing a connection for multiple origins can
result in requests being directed to the wrong origin server. For
example, TLS termination might be performed by a middlebox that uses
the TLS Server Name Indication (SNI) [TLS-EXT] extension to select
the an origin server. This means that it is possible for clients to
send confidential information to servers that might not be the
intended target for the request, even though the server has valid
authentication credentials.
A server that does not wish clients to reuse connections can indicate
that it is not authoritative for a request by sending a 421 (Not
Authoritative) status code in response to the request (see
Section 9.1.2).
9.1.2. The 421 (Not Authoritative) Status Code
The 421 (Not Authoritative) status code indicates that the current
origin server is not authoritative for the requested resource, in the
sense of [RFC7230], Section 9.1 (see also Section 10.1).
Clients receiving a 421 (Not Authoritative) response from a server
MAY retry the request - whether the request method is idempotent or
not - over a different connection. This is possible if a connection
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is reused (Section 9.1.1) or if an alternative service is selected
([ALT-SVC]).
This status code MUST NOT be generated by proxies.
A 421 response is cacheable by default; i.e., unless otherwise
indicated by the method definition or explicit cache controls (see
Section 4.2.2 of [RFC7234]).
9.2. Use of TLS Features
Implementations of HTTP/2 MUST support TLS 1.2 [TLS12] for HTTP/2
over TLS. The general TLS usage guidance in [TLSBCP] SHOULD be
followed, with some additional restrictions that are specific to
HTTP/2.
An implementation of HTTP/2 over TLS MUST use TLS 1.2 or higher with
the restrictions on feature set and cipher suite described in this
section. Due to implementation limitations, it might not be possible
to fail TLS negotiation. An endpoint MUST immediately terminate an
HTTP/2 connection that does not meet these minimum requirements with
a connection error (Section 5.4.1) of type INADEQUATE_SECURITY.
9.2.1. TLS Features
The TLS implementation MUST support the Server Name Indication (SNI)
[TLS-EXT] extension to TLS. HTTP/2 clients MUST indicate the target
domain name when negotiating TLS.
The TLS implementation MUST disable compression. TLS compression can
lead to the exposure of information that would not otherwise be
revealed [RFC3749]. Generic compression is unnecessary since HTTP/2
provides compression features that are more aware of context and
therefore likely to be more appropriate for use for performance,
security or other reasons.
The TLS implementation MUST disable renegotiation. An endpoint MUST
treat a TLS renegotiation as a connection error (Section 5.4.1) of
type PROTOCOL_ERROR. Note that disabling renegotiation can result in
long-lived connections becoming unusable due to limits on the number
of messages the underlying cipher suite can encipher.
A client MAY use renegotiation to provide confidentiality protection
for client credentials offered in the handshake, but any
renegotiation MUST occur prior to sending the connection preface. A
server SHOULD request a client certificate if it sees a renegotiation
request immediately after establishing a connection.
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This effectively prevents the use of renegotiation in response to a
request for a specific protected resource. A future specification
might provide a way to support this use case.
9.2.2. TLS Cipher Suites
The set of TLS cipher suites that are permitted in HTTP/2 is
restricted. HTTP/2 MUST only be used with cipher suites that have
ephemeral key exchange, such as the ephemeral Diffie-Hellman (DHE)
[TLS12] or the elliptic curve variant (ECDHE) [RFC4492]. Ephemeral
key exchange MUST have a minimum size of 2048 bits for DHE or
security level of 128 bits for ECDHE. Clients MUST accept DHE sizes
of up to 4096 bits. HTTP MUST NOT be used with cipher suites that
use stream or block ciphers. Authenticated Encryption with
Additional Data (AEAD) modes, such as the Galois Counter Model (GCM)
mode for AES [RFC5288] are acceptable.
The effect of these restrictions is that TLS 1.2 implementations
could have non-intersecting sets of available cipher suites, since
these prevent the use of the cipher suite that TLS 1.2 makes
mandatory. To avoid this problem, implementations of HTTP/2 that use
TLS 1.2 MUST support TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256
[TLS-ECDHE] with P256 [FIPS186].
Clients MAY advertise support of cipher suites that are prohibited by
the above restrictions in order to allow for connection to servers
that do not support HTTP/2. This enables a fallback to protocols
without these constraints without the additional latency imposed by
using a separate connection for fallback.
10. Security Considerations10.1. Server Authority
A client is only able to accept HTTP/2 responses from servers that
are authoritative for those resources. This is particularly
important for server push (Section 8.2), where the client validates
the PUSH_PROMISE before accepting the response.
HTTP/2 relies on the HTTP/1.1 definition of authority for determining
whether a server is authoritative in providing a given response, see
[RFC7230], Section 9.1. This relies on local name resolution for the
"http" URI scheme, and the authenticated server identity for the
"https" scheme (see [RFC2818], Section 3).
A client MUST discard responses provided by a server that is not
authoritative for those resources.
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Internet-Draft HTTP/2 July 201410.2. Cross-Protocol Attacks
In a cross-protocol attack, an attacker causes a client to initiate a
transaction in one protocol toward a server that understands a
different protocol. An attacker might be able to cause the
transaction to appear as valid transaction in the second protocol.
In combination with the capabilities of the web context, this can be
used to interact with poorly protected servers in private networks.
Completing a TLS handshake with an ALPN identifier for HTTP/2 can be
considered sufficient protection against cross protocol attacks.
ALPN provides a positive indication that a server is willing to
proceed with HTTP/2, which prevents attacks on other TLS-based
protocols.
The encryption in TLS makes it difficult for attackers to control the
data which could be used in a cross-protocol attack on a cleartext
protocol.
The cleartext version of HTTP/2 has minimal protection against cross-
protocol attacks. The connection preface (Section 3.5) contains a
string that is designed to confuse HTTP/1.1 servers, but no special
protection is offered for other protocols. A server that is willing
to ignore parts of an HTTP/1.1 request containing an Upgrade header
field in addition to the client connection preface could be exposed
to a cross-protocol attack.
10.3. Intermediary Encapsulation Attacks
HTTP/2 header field names and values are encoded as sequences of
octets with a length prefix. This enables HTTP/2 to carry any string
of octets as the name or value of a header field. An intermediary
that translates HTTP/2 requests or responses into HTTP/1.1 directly
could permit the creation of corrupted HTTP/1.1 messages. An
attacker might exploit this behavior to cause the intermediary to
create HTTP/1.1 messages with illegal header fields, extra header
fields, or even new messages that are entirely falsified.
Header field names or values that contain characters not permitted by
HTTP/1.1, including carriage return (ASCII 0xd) or line feed (ASCII
0xa) MUST NOT be translated verbatim by an intermediary, as
stipulated in [RFC7230], Section 3.2.4.
Translation from HTTP/1.x to HTTP/2 does not produce the same
opportunity to an attacker. Intermediaries that perform translation
to HTTP/2 MUST remove any instances of the "obs-fold" production from
header field values.
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Internet-Draft HTTP/2 July 201410.4. Cacheability of Pushed Responses
Pushed responses do not have an explicit request from the client; the
request is provided by the server in the PUSH_PROMISE frame.
Caching responses that are pushed is possible based on the guidance
provided by the origin server in the Cache-Control header field.
However, this can cause issues if a single server hosts more than one
tenant. For example, a server might offer multiple users each a
small portion of its URI space.
Where multiple tenants share space on the same server, that server
MUST ensure that tenants are not able to push representations of
resources that they do not have authority over. Failure to enforce
this would allow a tenant to provide a representation that would be
served out of cache, overriding the actual representation that the
authoritative tenant provides.
Pushed responses for which an origin server is not authoritative (see
Section 10.1) are never cached or used.
10.5. Denial of Service Considerations
An HTTP/2 connection can demand a greater commitment of resources to
operate than a HTTP/1.1 connection. The use of header compression
and flow control depend on a commitment of resources for storing a
greater amount of state. Settings for these features ensure that
memory commitments for these features are strictly bounded.
The number of PUSH_PROMISE frames is not constrained in the same
fashion. A client that accepts server push SHOULD limit the number
of streams it allows to be in the "reserved (remote)" state.
Excessive number of server push streams can be treated as a stream
error (Section 5.4.2) of type ENHANCE_YOUR_CALM.
Processing capacity cannot be guarded as effectively as state
capacity.
The SETTINGS frame can be abused to cause a peer to expend additional
processing time. This might be done by pointlessly changing SETTINGS
parameters, setting multiple undefined parameters, or changing the
same setting multiple times in the same frame. WINDOW_UPDATE or
PRIORITY frames can be abused to cause an unnecessary waste of
resources.
Large numbers of small or empty frames can be abused to cause a peer
to expend time processing frame headers. Note however that some uses
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are entirely legitimate, such as the sending of an empty DATA frame
to end a stream.
Header compression also offers some opportunities to waste processing
resources; see Section 8 of [COMPRESSION] for more details on
potential abuses.
Limits in SETTINGS parameters cannot be reduced instantaneously,
which leaves an endpoint exposed to behavior from a peer that could
exceed the new limits. In particular, immediately after establishing
a connection, limits set by a server are not known to clients and
could be exceeded without being an obvious protocol violation.
All these features - i.e., SETTINGS changes, small frames, header
compression - have legitimate uses. These features become a burden
only when they are used unnecessarily or to excess.
An endpoint that doesn't monitor this behavior exposes itself to a
risk of denial of service attack. Implementations SHOULD track the
use of these features and set limits on their use. An endpoint MAY
treat activity that is suspicious as a connection error
(Section 5.4.1) of type ENHANCE_YOUR_CALM.
10.5.1. Limits on Header Block Size
A large header block (Section 4.3) can cause an implementation to
commit a large amount of state. In servers and intermediaries,
header fields that are critical to routing, such as ":authority",
":path", and ":scheme" are not guaranteed to be present early in the
header block. In particular, values that are in the reference set
cannot be emitted until the header block ends.
This can prevent streaming of the header fields to their ultimate
destination, and forces the endpoint to buffer the entire header
block. Since there is no hard limit to the size of a header block,
an endpoint could be forced to exhaust available memory.
An endpoint can use the SETTINGS_MAX_HEADER_LIST_SIZE to avise peers
of limits that might apply on the size of header blocks. This
setting is only advisory, so endpoints MAY choose to send header
blocks that exceed this limit and risk having the request or response
being treated as malformed. This setting is advertised hop-by-hop,
so any request or response could encounter a hop with a lower,
unknown limit. An intermediary can attempt to avoid this problem by
passing on values presented by different peers, but they are not
obligated to do so.
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A server that receives a larger header block than it is willing to
handle can send an HTTP 431 (Request Header Fields Too Large) status
code [RFC6585]. A client can discard responses that it cannot
process. The header block MUST be processed to ensure a consistent
connection state, unless the connection is closed.
10.6. Use of Compression
HTTP/2 enables greater use of compression for both header fields
(Section 4.3) and entity bodies. Compression can allow an attacker
to recover secret data when it is compressed in the same context as
data under attacker control.
There are demonstrable attacks on compression that exploit the
characteristics of the web (e.g., [BREACH]). The attacker induces
multiple requests containing varying plaintext, observing the length
of the resulting ciphertext in each, which reveals a shorter length
when a guess about the secret is correct.
Implementations communicating on a secure channel MUST NOT compress
content that includes both confidential and attacker-controlled data
unless separate compression dictionaries are used for each source of
data. Compression MUST NOT be used if the source of data cannot be
reliably determined.
Further considerations regarding the compression of header fields are
described in [COMPRESSION].
10.7. Use of Padding
Padding within HTTP/2 is not intended as a replacement for general
purpose padding, such as might be provided by TLS [TLS12]. Redundant
padding could even be counterproductive. Correct application can
depend on having specific knowledge of the data that is being padded.
To mitigate attacks that rely on compression, disabling or limiting
compression might be preferable to padding as a countermeasure.
Padding can be used to obscure the exact size of frame content, and
is provided to mitigate specific attacks within HTTP. For example,
attacks where compressed content includes both attacker-controlled
plaintext and secret data (see for example, [BREACH]).
Use of padding can result in less protection than might seem
immediately obvious. At best, padding only makes it more difficult
for an attacker to infer length information by increasing the number
of frames an attacker has to observe. Incorrectly implemented
padding schemes can be easily defeated. In particular, randomized
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padding with a predictable distribution provides very little
protection; similarly, padding payloads to a fixed size exposes
information as payload sizes cross the fixed size boundary, which
could be possible if an attacker can control plaintext.
Intermediaries SHOULD retain padding for DATA frames, but MAY drop
padding for HEADERS and PUSH_PROMISE frames. A valid reason for an
intermediary to change the amount of padding of frames is to improve
the protections that padding provides.
10.8. Privacy Considerations
Several characteristics of HTTP/2 provide an observer an opportunity
to correlate actions of a single client or server over time. This
includes the value of settings, the manner in which flow control
windows are managed, the way priorities are allocated to streams,
timing of reactions to stimulus, and handling of any optional
features.
As far as this creates observable differences in behavior, they could
be used as a basis for fingerprinting a specific client, as defined
in Section 1.8 of [HTML5].
11. IANA Considerations
A string for identifying HTTP/2 is entered into the "Application
Layer Protocol Negotiation (ALPN) Protocol IDs" registry established
in [TLS-ALPN].
This document establishes a registry for frame types, settings, and
error codes. These new registries are entered into a new "Hypertext
Transfer Protocol (HTTP) 2 Parameters" section.
This document registers the "HTTP2-Settings" header field for use in
HTTP; and the 421 (Not Authoritative) status code.
This document registers the "PRI" method for use in HTTP, to avoid
collisions with the connection preface (Section 3.5).
11.1. Registration of HTTP/2 Identification Strings
This document creates two registrations for the identification of
HTTP/2 in the "Application Layer Protocol Negotiation (ALPN) Protocol
IDs" registry established in [TLS-ALPN].
The "h2" string identifies HTTP/2 when used over TLS:
Protocol: HTTP/2 over TLS
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